A/N : Written for an anon requester as a Christmas request giftfic. The request was for something based on this quote: "The reason I know what we are to each other is because we fight freely and almost constantly, about even the smallest thing. In fact, once we didn't speak for an entire week because he didn't like the way I loaded his dishwasher…I can't decide if we're exact opposites, or somehow exactly the same except for minor cosmetic differences. I do know that all of his friends hate me and all of my friends hate him. We drive each other crazy in ways that nobody else can even touch. We never bore each other. And we both realize what a rare thing this is." — Augusten Burroughs (Dry)

Introspective relationship fic.

The Gain-Loss Theory of Attraction

Izaya always wakes first.

He resolutely refuses to do anything so cozy as sit down over breakfast with monsters, and so it's always easier to be up and dressed and out of the door before said monster stirs from its' fathomless sleep and staggers out of its cave.

Fathomless enough that Shizuo always manages to sleep through the cataclysmically loud cranking and wheezing of poorly installed and barely maintained plumbing while Izaya showers. Weren't wild animals supposed to have exceptional hearing? He even hums loudly while he suffers through the sputtering spray, but when he's finished and the pipes have settled to a dull roar, he can still hear the obnoxious sound of Shizu-chan's snoring past it all. The snuffly sound drifts from the tiny bedroom of a tiny apartment, and Izaya is thoroughly irritated by the time he's dressed because he doesn't know whether he wants to go back in there to smother Shizuo with his own pillow or just crawl back under the sheets.

The apartment is a mess, but he didn't make it so he refuses to pick it up. He didn't knock that lamp over, for instance. Well, his ass might have, but technically Shizu-chan just had very poor aim when he swung them too close on the way the bedroom. He remembers something crashing at the time, past the sound of shirts tearing and desperate panted groans. He picks up his clothes, left in a trail between the front door and Shizuo's bed, a reverse-action replay of last night as he picks his way like a cat over the wrinkled pools of cheap fabric that mark where that bartender uniform met its demise. There's a tear in one shoulder of his sweater. He pokes at it, irritated, before humming softly to himself as he picks Shizuo's shirt up as he saunters to the kitchen, tearing a similar rip in the shoulder before tossing it jauntily over his shoulder.

He hates instant coffee, but he helps himself to a cup because it's the only thing Shizu-chan has. He hates it when they end up back here – it's not his place, and these aren't his things. And stealing the remainder of Shizuo's milk to turn his coffee extra creamy won't change that, but it makes him feel better.

By the grainy morning light, he makes his way to the living room, where Shizuo's cigarettes have been abandoned and forgotten on the cheap coffee table. He prides himself on being enough of a distraction that the blond hadn't even remembered to take them into the bedroom with him last night. It's tacky, playing second fiddle to Shizuo's first love, but he doesn't delude himself that any of his scathing remarks have any impact. He could drop a tar-black lung on the breakfast table and Shizuo would keep on smoking just to spite him.

He takes the cigarette and the lighter anyway, crumpling them into the pocket of his coat along with his knife and his phone. It won't do Shizu-chan any harm to do without for the day, and as often as he's here, Izaya suspects his coat stinks of cigarette smoke now anyway.

With the sound turned down, he flicks idly through the news channels as he sips his coffee. He could turn it back to the channel Shizuo left it on, but he wrinkles his nose over the top of his cup at some protozoic sports channel with nonsensical stats and grown men grappling. He's never known anyone with such dumb taste, he really hasn't. It's like sinking down to the level of a twelve year old.

Maybe he used up all his good taste on me, ne?

He turns the TV to a documentary channel instead, before switching it off. It'll do Shizu-chan good to be learning something now and then.

Rinsing the coffee cup out in the sink, he leaves it upright on the rack to dry. Of course cups go upright; why on earth would you rest the rim – the part where your mouth went – on grimy plastic that had seen the grubby bottom of too many plates Shizuo had a habit of leaving on the floor as makeshift ashtrays? That made no sense. He might as well gnaw on the carpet and be done with it.

Satisfied he's done his bit to bring healthy living, culture, education and good kitchen hygiene to Shizu-chan's hopeless little existence, he lets himself out, locking the door behind him with the key Shizuo's given him.

Izaya always wakes first.

Shizuo thinks the damn flea does it just to piss him off, like he does everything else. He hasn't worked out exactly why – yet – but decides that it's either 'cause the louse is only really good for fucking and running, or it's some kind of lofty judgement call on the fact Shizuo can roll out of bed late because he's got the luxury of a dead-end job he can do in his sleep.

And he knows the bastard's used up most of his hot water. Not all of it – nothing's ever that fricking simple with that damn louse. There's always that ten seconds of blissful warmth pelting his skin, lulling him into a false sense of security. Sometimes he's even stupid enough to think maybe Izaya's suddenly developed the skill of giving a shit about other people, before the water turns cold, peppering his skin with icy daggers.

Kinda feels like the summation of his relationship with the flea, he thinks, cursing under his breath. Starts off hot, somewhere in the middle he starts thinking dumb, impossible things, and it ends up fricking painful with a whole lot of him wishing Izaya was within punching distance.

Grumbling to himself as he stalks out of the bathroom, one wet foot catches in the pocket of his pants, left conveniently right in front of the door. Grunting, he thrusts out a hand to brace himself as he trips face-first towards the opposite wall, only for the impact to leave a crater in the plaster, a wispy spiderweb of cracks radiating from its centre. Fuck it, he'd been so careful last night not to damage the walls again. A lamp has fallen over; so has a chair, one of the couch cushions, and, weirdly, his blinds are all rucked up and tangled even though he can't even remember slamming Izaya into those. Scowling to himself, he picks his clothes up as he goes, on an even keel until he picks up his shirt, and his fingers go right through a ragged hole in the shoulder. He stares at it for a second, as if his glare could learn to sew, before huffing out a sigh and ditching the clothes on the couch.

His mood doesn't improve when he opens the fridge and reaches for the milk carton, only to find about a quarter of an inch sloshing around in the bottom.

"Goddamn shitty louse…!"

He tried tipping the remainder out, but by the time it's clung to the inside of the carton there's barely a trickle of milk against his tongue. No doubt Izaya would have some smartass remark to say about how only brutes drank out of the carton.

Yeah, and only assholes use other people's milk without asking!

If Izaya ever did ask, Shizuo was gonna say "Yeah, sure," just to screw with him.

He growls as he rights the cup on the rack by the sink. Who the hell leaves cups upright to dry? Where the hell is the water supposed to go? And unless they're tipped onto the rim, the soapy residue from the detergent – and damn it, Izaya always uses too much, like he's trying to kill every fricking bacteria in the apartment, not just the coffee cup – dries at the bottom and makes everything taste like a Laundromat. Seriously, he thought Izaya was supposed to be smart?

He doesn't bother with the TV. Last time Izaya'd stayed over, he'd turned it on the next morning after the flea left, only to find the channels tuned to some cheesy pop station. And girl bands with what looked like fifty fricking three members shrieking at him first thing in the morning was not on his agenda again. 'Specially when Izaya had left the volume maxed out, ensuring that most of the street shared his horror. Fuck it, he was still getting those 'secretly, I'm questioning your masculinity' looks from his neighbors.

There's gotta be some kinda factory reset on the thing. Even if it means he has to manually retune every channel, it's better than finding out what treat the louse has left for him today. 'Sides, Kasuka isn't on any show this week, so he has the time to do it.

And he almost gets out unscathed.

Dressed, he's already pissed off at the thought of having to stop somewhere for breakfast now Izaya's stolen his. It's not just that eating out all the time costs money he doesn't have – regular people don't splurge on fancy tuna every other minute; he bets Izaya doesn't even use the half-price coupons on principle – but damn it he wanted his milk. Just about the only concession he makes to being practical and responsible is making sure there's plenty in the fridge, and the shitty flea has to use it up…

He reaches for his cigarettes on the coffee table as he feels the anger rising. Sometimes he doesn't even need the nicotine; holding the packet and knowing it's in there is enough.

The cigarettes are gone. So is his lighter.

"Goddamnit…" His hands clench into fists at his sides, and somewhere, cowering under their kitchen tables, his neighbours must be wishing they had the population-of-a-small-country girl bands back when he roars, "…Goddamn shitty louse! I'll fucking KILL YOU!"

"I can't stand him."

Spinning back and forth in his swivel chair, Izaya taps an off-beat rhythm on the edge of the desk with the tip of his knife.

"I mean, he doesn't even deign to be remotely human so that I can love him along with everyone else. He doesn't do anything I want him to do…" Lips pursed, he worries at a chip in the veneered edging of the desk with the point of the blade. "It's so inconvenient. Why can't he just die already? Did I tell you he leaves cups upside down to dry, isn't that the most irrational thing you've ever heard?"

Namie doesn't even look up. Just glances, rather bored, at her wristwatch and says, "Seven times."

Izaya stops spinning. "Hmm?"

"You've mentioned him seven times this morning. And it's the second time you've mentioned the cups. I don't care any more now than I did the first time, in case you were interested."

"But it's still thoughtful that you're keeping track." He smiles, attention drifting back to the pressing task of hacking tiny corners of paper from the documents on his desk. It's nothing important. Now and then… well, more often now, he instructs Namie to make some arbitrary number of print-outs or photocopies, just for fun. "It saves me the effort, ne?"

"It would be if you bothered doing anything productive with your time."

"I am doing productive things." He starts spinning again. "I'm telling you why I can't stand Shizu-chan."

"So why don't you kill him? You're so much smarter, he's a dumb animal, blah, blah, blah…"

"I could," Izaya agrees, "but that'd be cutting off my nose to spite my face, ne? I'm a busy man. I don't have time to cultivate another outlet for my entertainment needs. Well, not one that good, anyway."

"Or maybe you can't." Namie smiles at her screen, flicking back a strand of hair over her shoulder. "Not just that you literally can't, because he'll smack your face into the nearest wall, but you can't, because then what would you have to whine about all the time?"

"Oh, I don't know… the lack of good secretarial help these days?"

"Just as well I'm not a secretary then, isn't it?"

"And I'm not whining. I'm just… sharing my thoughts."

Namie laughs. "Oh, really? You're doing that thing again, aren't you? Where you've mistaken me for someone who actually cares about your not-so-clandestine sex life. Or about your life, generally, in the 'is Izaya Orihara still breathing' sense."

Izaya arches a brow. "Just as well I'm not paying you for your people skills, ne?"

She shakes her head. "You barely pay me enough to make your tea, let alone make sense of your rambling."

"What about you, Namie?" Wheeling over to her desk, he leans his elbows on the edge, chin propped on his hands. "You hate Shizu-chan too, ne?"

"He hurt Seiji. Of course I hate him." His secretary pauses, looks up, stares out of the window for a second before shrugging and going back to work. "But then I can't stand you, either. It wouldn't be difficult, you know. Strong doesn't necessarily mean immortal. Have you given any thought to poisoning him?"

Izaya makes a face. Whenever he thinks of doing anything to Shizu-chan that backs up all his demands, he just thinks of how bored he'd be. Where among his humans could he ever find as perfect a foil as he's found in this monster? If Shizuo really was made for him – to be his opposite, and the closest thing to an imbalanced equal he's ever known – by some grand cosmic plan, then he thinks there was probably a glitch with the blueprint somewhere.

No, there's a reason fantasies remain fantasies. He doesn't think he'd be very happy if Shizu-chan really was another ordinary human. It would be harder to justify the… special treatment he singled the blond out for, were that the case.

Growing bored of trying to glean a reaction, and annoyed at Namie's unhelpful suggestions, he wheels back over to his desk. Standing to attention next to his keyboard is a half-full packet of cigarettes and a cheap plastic lighter. Chin resting on steepled fingers, he smiles.

"I think Shizu-chan is immune."

"I can't stand him."

With every stomp from the convenience store to where his colleagues wait for him on the street corner, Shizuo's irritation just grows.

"Can't stand him, can't stand him, can't stand him…"

Tom looks a little alarmed, while Vorona watches him with that stoic expression that might mean she's thinking of strawberry chiffon cake or, alternatively, how best to threaten their next deadbeat with horrifically imaginative violence. Shizuo always hopes it's cake, but…

Anyway, neither of them has to ask who he can't stand.

Tom sighs, hands in his pockets. "What happened this time?"

Lighting a cigarette and taking the first blissful drag, Shizuo wonders how far back into the story he'll have to rewind for it not to sound like Izaya fucked him over, literally and figuratively. If he explains the flea stole his cigarettes, it'll beg the question when and how he got close enough to even try. He coughs a little on the smoke, not sure whether it's funny or not, the thought that he could stow the cigarettes anywhere on his person and Izaya would always get close enough to try.

Hell… the flea had already been under Shizuo's skin by the time he decided to take a blade to it, and the mellowing he'd expected to happen with time never materialized. Probably stood to reason, when 'mellow' wasn't exactly an accurate description of either of them.

And he still doesn't know why. Why any of it. The closest thing he can pin down is that he doesn't do this with Izaya. Doesn't stop, or hesitate, or even for that split second imagine that anything he'll say will somehow be wrong. 'Cause it's been wrong from the start, and that's why it's right.

"Just pissed me off, that's all."

Tom sends him a sidelong look over the top of his glasses. "You okay?"

Shizuo thinks of torn shirts, and right-side-up coffee cups, and no milk or hot water. "As long as he stays the fuck out of my way, I will be."

"The likelihood of avoiding Izaya Orihara, very poor. The solution has been provided; the noxious insect can be eliminated. Simple, simple." Vorona nods, striding away with what he swears is a cheerful spring in her step even while her expression barely changes. "Further conversation, not required."

"Oi, no one's being eliminated, okay?"

Vorona looks back, confused. "The proposed solution is unsatisfactory? Please inform of a more suitable resolution to ensure Senpai and the noxious insect's trajectories do not intersect?"

"No, no, I didn't…" He blows out a thoughtful sigh of smoke. "Well, it's like those things you say, y'know? Like, 'I'd kill for a milkshake'. No one's really going to kill someone for a milkshake."

She's looking at him with that vague bewilderment that makes him dread that, one day, he's going to walk into a massacre at McDonalds. He really needs to be careful what he says – how he says it – around her. And at a loss on how to explain without confusing himself, he just ruffles her hair.

"I'm not that mad, really. And when I say stuff like that, I don't really mean it, so don't go getting in trouble for me okay?"

Vorona nods, but he still gets the impression she thinks he's weird for turning down the offer. Maybe he is, but…

Something about a life without being pissed off, without broken lamps, torn uniforms, mornings that turn into fricking obstacle courses and shit like hiding his cigarettes and making him think that Izaya actually does that stuff in some warped version of caring…

I'd be bored out of my mind.

"She's got a point, though…"

"Tom-san!"

"Oh, no, not the eliminating part." Tom shakes his head. "Just that there's got to be something you can do to avoid him."

Shizuo scoffs softly. "Yeah. I could move. Or die. Since I don't plan on doing either any time soon, I guess I gotta live with it."

And it wasn't that bad, really. The good shit made up for all the shit that annoyed him. When it was good, it was the kind of really good that obliterated everything else.

"Oi, Tom-san…?"

"Hmm?"

"How do you dry cups? Upside down, or right side up?"

Tom blinks. "Uh… upside down, I guess? I never really thought about it."

"The inverted cup allows gravity to encourage water away from the surface. This facilitates the drying process by distributing a thinner layer of moisture over a wider area," Vorona says, and he's relieved that all thoughts of elimination seem to be forgotten. "Very efficient."

"See?" he says as they head off to the next job, satisfied that he's right and Izaya's an idiot. "That's what I said."

Shizuo's not hard to find.

Izaya can't claim the monster's sixth sense for tracking people down in really creepy ways, but he usually finds the blond if he just follows the sweet frisson of violent chaos that permeates Ikebukuro. He knows he's getting closer when the air vibrates with it, raising the hair on the back of his neck like the promise of an encroaching storm.

Ah… maybe it is some sort of sixth sense, after all. And he's well aware that the storm will only chew him up and spit him out, but the ride is eternally worth it.

The blond is standing outside a pachinko parlour, face upturned, sunglasses shading his eyes. Izaya wonders briefly where Shizu-chan's guard dogs are this afternoon, before deciding it doesn't matter. If their neglect for Shizuo's welfare makes life easier for him, who is he to complain?

"Shi~zu-chan!"

The cigarette perched between Shizuo's fingers – of course he'd find more, Izaya had expected that – snaps like a twig.

"IIIIZAYAAA!"

He stopped off at a pharmacy on the way here. Judging by the look of homicidal rage on Shizu-chan's face as he thunders in Izaya's direction, it was a good idea; subtle only aggravates primitive beasts. Their little brains are far better suited to the direct approach. Pulling the box out of his pocket, he spins it like a Frisbee in Shizuo's direction, giggling at the way the blond dodges like he was expecting flying shuriken instead.

"The fuck is this?"

Izaya pivots on one heel as he prepares to run, hesitating for just a moment to see the beast's reaction when Shizuo leans down to pick up the box. "Nicotine patches, ne?" He shrugs, feeling the shushed rise and fall of his coat, the fur trim swishing. "I'm tired of having all my clothes stink of smoke. Not to mention, smokers are horribly unattractive."

"…who the fuck says you need to even breathe the same air as me, hah?"

Izaya tilts his head, smiling back over his shoulder before he takes off at a run. "It's hard to avoid it when you're moaning into my mouth, ne?"

"Goddamn flea…!" The box whizzes past his head, moving even faster than he can run. Behind him, he hears the familiar thud of Shizuo's footfalls, mowing down the distance between them with every long stride. "Get the hell back here so I can kick your ass!"

"Kick it, Shizu-chan?" Izaya tosses back flippantly, sidestepping the trashcan that lands in his path, hopping over it, still running. "That's not what you wanted to do to it last night!"

"Shut the fuck up!"

Izaya laughs, feeling the wind buffet his coat as he runs. There's a city street under his feet, adrenaline singing in his blood, and Shizuo at his back, and in the face of something so visceral there's a space where the line between humans, monsters and gods is chalk-thin and faded. There's just this, and he's not even sure why he thinks he wants anything else.

They've forgotten why it started, or whether there was even a reason at all. All he knows is that this is as intrinsic and necessary a part of his life as breathing, and ootoro, and loving all humans equally – but none in particular. Now it's back to the same jaded insults and the brand new catalogue of wrongs, neither of which are thrown with any accuracy. Neither of them truly wants to land a hit. Why on earth would they, when that would destroy all the fun they're having?

He runs, because Shizu-chan will chase him. Shizu-chan chases him because he runs. It's a perfectly played out dance, tangled in its simplicity, and he supposes he can't blame anyone else for not understanding its facets and nuances.

They do. They know the point to the chasing and the fighting and the bickering.

Nothing else matters but that.

Shizuo's not hard to find.

He knows that. Hasn't he made a point of standing out, a black and white and gold warning signal for any asshat dumb enough to cross his path?

But there's still always an indignant flare of annoyed surprise whenever he realizes Izaya's tracked him down anyway. Annoyed surprise, and… something that makes him breathe a little easier. With all the shit Izaya gets up to, he gets uneasy if the flea stays away too long. Of course, the dumbass ruins the moment the second he opens his mouth. Shizuo gets the feeling he should be used to that by now.

He spares the box of nicotine patches a thought as he gives instinctive chase, a hunting dog who's caught sight and scent of the sly fox. Those things aren't cheap; he knows from the last time he bothered checking, right around the time Izaya started whining about the cigarettes. He'll go back when he's done here and see if they're still there…

And then plaster 'em all over his mouth to shut him up!

Pedestrians scatter in the face of their running battle. Izaya dodges everything he throws, so he sacrifices a few paces for the detour that takes him to the stop sign on the corner of the street. Metal creaks at the base of the pole twists and warps its way free of the concrete, the air whistling around it as he angles one swing of it at the flea's feet.

"If you want me to stop…" Izaya laughs, easily skipping over it. Shizuo thinks it's 'cause he's not close enough, and ups his speed. "Then you could always try asking nicely, ne? I know you're capable of it."

"Like you ask nicely before you use up all my shit? And goddammit, cups go right side down to dry, shitty dumbfuck louse!"

"Che, no they don't." He can't be sure if he speeds up or if Izaya slows down, but somehow they find themselves at the perfect distance again, adjusting and compensating for the other's moves. "Really, Shizu-chan, and I thought my efforts at correcting you were having some effect…"

"You mean…" Another sharp swing. Izaya seems to fly as he leaps back, airborne for just a fraction of a second too long, launching himself off the side of a brick wall for leverage to pull away. "Efforts to piss me off!"

"Only because you're averse to progress, Shizu-chan! And to not living like something that crawled from a swamp."

Averse to progress, my ass… I'm with you, aren't I?

On the next swing, the edge of the sign clips Izaya's heel, and while he doesn't trip, the flea still has to brace on one knee when he lands awkwardly.

"Hah! That's for using my milk, goddamn louse!" Another swing forces Izaya to duck. "And that's for using my hot water!"

Izaya glares back, and the next thing he knows he's got to take a handful of skipped steps back himself, or the edge of that knife is gonna ruin another shirt. Izaya's eyes are blazing over the outstretched hand, the blade gleaming in his hands.

"Is it my fault you can't shop?"

"Who the hell can't shop?" There's a cascade of sparks as street sign meets flick blade, neither giving way. "You don't even like milk, why the hell are you drinking it all?"

"I like Shizu-chan's milk." In a blink, the glare is replaced by an effortless smile. Holding the knife loosely, aimed at nothing in particular, Izaya shrugs. "It's special."

"Disgusting fricking flea…!" With a roar he charges, sign held out like they're jousting knights. And he knows Izaya'll sidestep, but damn it, he hadn't noticed they were on one of the pedestrian bridges over the highway.

There's a look of amused alarm in those red eyes as Shizuo tries in vain to back-pedal, but even he isn't stronger than gravity, and the safety rail doesn't do much to restrain him.

It doesn't hurt him, obviously, but he's glad the vehicles heading his way manage to swerve all the same. He doesn't need Izaya spending the next month giggling at all the broken, dented fenders he'd have to pay for. Pile up on Sunshine Street. Local ex-bartender goes broke from repair bills. Blames goddamn shitty louse for never knowing when to quit. Amid the honking horns and squeals of brakes, he picks himself up and checks his uniform for tears before tossing the sign aside with a clatter. It looks in tact. So the flea gets to live.

Lifting his head, he scowls at the figure peering down at him over the edge of the bridge. "You're fricking paying for this, you know that right?"

Izaya leans on the railing and smiles down at him. "Your place or mine, Shizu-chan?"

And for all the fucking hassle the flea causes, there's only one answer.

"Mine."

Izaya's waiting outside Shizuo's door with a carton of milk.

Shizuo trudges up the landing, still adjusting his shirt cuff from applying one of the nicotine patches.

"Gonna drink all that by yourself?"

"Ah, didn't I tell you? I don't even like milk."

"Tch…"

They don't get as far as the fridge. For a good couple of hours, the milk carton stays on the entryway floor, half-hidden under the tangled sleeves of Izaya's coat and Shizuo's shirt.

Later, Shizuo might pad out of the bedroom, barely dressed and scratching at the old scar on his chest as he digs around their rumpled clothes for his cigarettes. Because good sex followed by a nicotine patch doesn't really cut it, and between the sweat and Izaya's wandering hands, the patch is long gone anyway.

And maybe later still, Izaya might slink out, smiling idly to himself as he empties most of the milk into a dozen different cups and glasses he hides at the back of the fridge, leaving just an inch at the bottom of the carton. Because humans might be humans, but there's nothing more breathtakingly alive than a pissed off beast.

They're two jigsaw pieces. No one's sure anymore whether they even came from the same box, let alone the same puzzle, but they fit together so seamlessly, no one can see the join.