A/N: If I needed any more reasons not to be on Twitter... Proud TV Junkie posted this prompt: "Cornered in the break room by Beckett after seeing her kiss Demming in the hall, Castle comes out and asks, why him? Why not me?" I decided to run with it. Hope you enjoy.

Why Not Me?

Chapter 1

He catches just a glimpse in the shadow of a doorway halfway down the hall, but that's more than enough. He's seen the eye contact between the two of them since they've all been working together, the subtle and not so subtle flirting that he doesn't know how to handle. And now they're kissing. In the precinct. The place he thinks of as his second home. The place he thinks of as theirs. The place where he feels he has the most value in the world, other than when he's looking into his daughter's eyes.

He darts out of sight, hiding up against a wall like a fugitive, counting his breaths to get his anger and his deep, deep disappointment under control. When he hears them parting, he ducks into the break room. He can pretend to get a coffee then leave after they have left without having to come face-to-face with Kate.

He's fake-brewing a cup when the door opens and lord if it isn't Beckett with a sweet, secret smile on her face. "Castle. What are you still doing here?" she asks.

"Hey," he says with false levity, trying not to snap the handle off the cup. "Just about to leave myself. Good job closing the case." His voice is scratchy, but she doesn't seem to notice.

She smiles at this offer of congratulations and doesn't question it when he abandons the coffee cup he'd been toying with. "Yeah. Well, we make a good team."

That's what he thought: that they make a good team, that they might go further than that. "Yeah. Good night," he says, feeling all hope drain away now that she's chosen Demming. She's a cop. Maybe he never stood a chance. Maybe she was only ever going to choose a cop. Maybe his pipe dream of them going all the way was a stupid, self-deluding fantasy.

Beckett, oblivious to all of this mental turmoil, simply says, "Thanks. Night."

Castle turns and hurries away. But as he passes her desk and his chair, fresh questions boil inside his brain. He's assaulted by images of past moments when they seemed so close. The looks, the smiles, the thousand little touches, glances and sentence-finishing instances that said they were bound for great things as more than partners. If only they knew how to get from point A to point B. If only they could get out of their own way.

Castle listens to the elevator rising with a cheated kind of fury boiling in his blood. Demming swooped in and stole what was going to be his. He doesn't deserve her, he doesn't know her, he⏤

The break room door slams so hard that it sounds like a gunshot going off. Kate physically startles at the ricochet of wood and glass inside the frame. Her mug clatters into the sink and she spins around, fast-draw reaching for her belt and the sidearm that is…

Sitting in her desk drawer. Fuck!

When her gaze settles on the source of the noise, she quickly realizes that the threat is minimal, which is a good thing now she's armed with only a teaspoon. After reverberating for several seconds, the door stops juddering and now stands slightly ajar. She finds herself standing face-to-face with Castle. Drama over she thinks. But the longer she looks, the more she begins to reassess. Her partner's eyes are blazing with fury. His whole body is stiff with what looks like either rage or frustration.

"Castle, what the hell?" Kate says. She's embarrassed that he most likely witnessed her freaking out. She's always been impervious to sudden loud noises. In this job and in this city you pretty much have to be or you'd end up a nervous wreck.

Without waiting for a reply, she turns her back on him to focus her attention on the sink, where she recommences rinsing her discarded coffee mug.

Castle doesn't say anything at first, and for a moment, while the faucet runs and her mind strays back to the case, she forgets that he is even in the room with her. So when he clears his throat and shifts his feet, the grating sound of grit trapped between the floor tiles and the leather soles of his shoes has her startling for a second time. She dumps the dishtowel on the side of the sink and spins around to face him.

"What are you still doing in here? And what's with the creepy silent treatment? Did you need something from me?"

Oh, he needs something all right.

Castle puts his hands on his hips and his jacket falls away on either side of his body, putting a pretty hard looking set of abs, a lean ribcage, and a well-defined waist on display. He doesn't necessarily mean to do it, but Kate can't help but see. And by see, she means stare.

"Why him and not me?"

Kate frowns. "Hmm?"

"I said…why him and not me?" Castle repeats more firmly.

"I heard you the first time. Raising your voice doesn't make it any clearer." She turns her back again because although she isn't a hundred percent certain what he's talking about, she's a detective and she has her suspicions.

"Demming." Castle drops the D-bomb on the floor, lets it roll and then detonate.

Kate tosses the dishcloth to one side and braces both hands on the sink. Shit. She's right. He's going there. No way she's turning around now.

"What…what are you getting at Castle? What about Demming?" she bluffs, badly.

"I saw you two kissing just now. Don't even think about denying it. I'm sure there were plenty of other witnesses around. You weren't exactly being discreet."

Kate releases a pathetic huff of sound; she doesn't have a leg to stand on. He caught her kissing Tom Demming; she has no defense. It doesn't even occur to her to challenge Castle at this point. To demand why he thinks it has anything to do with him who she kisses. But just as he knows, so does she; there is a double standard here. She just doesn't want to face up to it.

So she goes on the offensive.

"How dare you pry into my private life. As if the creepy staring wasn't bad enough, now you're skulking around corners spying on me?"

Castle doesn't say a word. The man she usually can't shut up has suddenly turned into the strong silent type. Great. Now she doesn't even know if he's left the room. She's going to have to turn around and face him. Look him in the eye and look at that broad, tall, hard body of his that she should not be thinking about. Not one iota. Certainly not now that she's with Tom.

So why can't she help herself? And why is she kissing another guy at work, and only when Castle's around? She's never behaved so unprofessionally in the past, no matter whom she was dating. Kissing another cop when anyone on her team could see her is just…it's insane. Kissing him when Castle could see…well, that's just cruel. She knows the guy likes her; he made that clear from day one. And over time that liking has taken on a different color from the arrogant lustful 'I can have any woman I want but I'm choosing you' variety of their early days.

Argh! Kate's utterly unprepared to look under that rock, so on the defensive, she goes.

"Why are you still here?" she demands again, this time to Castle's face.

Some of his anger seems to have dissipated if his face is any indication. But there's still something like hurt or wounded pride hardening his…beautiful blue eyes.

"Why him, Beckett? Why not me?"

Sweet Jesus.

"Castle," she sighs. "Don't…just don't go there. Okay. We don't need this. Not right now."

He shakes his head. "I disagree."

"There's a surprise," she mutters, crossing her arms over her chest.

"Be as sarcastic as you like. I want to talk about it. So why don't you explain it to me, Beckett? I've been here for two years, following you around–"

"No one asked you to. In fact, I expressly asked you not to," she counters.

Castle ignores her, barrelling on with his list. "Helping you solve cases." This earns a scoffing sound. "Bringing you coffee."

"I wasn't aware there was a charge for the coffee and the crazy theories," she snipes back. "And I wasn't aware that you were hanging around expecting some kind of…of relationship tenure," she spits out.

The room falls silent. Kate looks down at her feet. As angry as she is with Castle for cornering her, she hates herself more.

She presses a hand to her flushed cheek. "I…I'm sorry. I shouldn't have said that. I appreciate everything you do to make my job easier, Castle. I do."

He waves her apology aside. "Noted. But my question still stands. Why him and not me?"

Kate hates being put on the spot. Especially when Castle has something of a point. Why did she allow Tom Demming into her life, if not her bed (yet), when Castle's been trying to tunnel his way in for the last two years and all he's got to show for his loyalty are a few attaboys and the odd burger at Remy's?

"We…we work together," she says lamely.

"Not good enough. You're working with him, too," Castle points out.

Finally, she's had enough. "I do not have to justify my personal life…my personal choices to you. I don't browbeat you when you date another…bimbo," she yells.

Castle nods to himself, a terse smile forming on his lips. "Great. I see where we stand. You get to date detectives while anyone I date is automatically a bimbo. Nice strike for equality there, Beckett. I think I've heard enough. He's your type. I'm obviously…not. Let's leave it at that. It was nice working with you, Detective Beckett. Good luck for the future."

Kate is stunned when Castle turns and heads for the door. In a moment of panic, she raises her voice.

"You want to know why not you? Fine. I'm too scared to let myself get close to anything real. Okay? So there…there you have it."

Castle spins around. His face shows his interest in her Hail-Mary remark, but he's hanging on by the skin of his teeth.

"What's Demming then if not real? Because I know what I saw. That kiss was no figment of my imagination."

Kate fumes at being forced to spell it out. "He's…he's fun. Okay? Uncomplicated. That's…" She lowers her voice and looks away. "That's all I can handle right now. I'm sorry."

"You're sorry? Why are you sorry?"

She looks down at the floor again. "I never meant to hurt you."

Castle shrugs. Her apology has such a ring of finality to it. This whole thing is a done deal. He needs to move on. "Like you said. It's your life. I shouldn't be poking my nose in…again. I had no right demanding anything from you. I'm sorry, too." He turns away again to leave for good this time.

Kate takes a step towards him. "Castle, please don't go. We make a good team…a good partnership. This job is hard. It's more fun working with you."

He sounds tired and resigned when he says, "Kate, I can't be your plaything anymore. And I can't stand around while you date other guys right under my nose. Not when you know very well how I feel about you."

TBC...

A/N: Chapter 2 is written and will be posted in a couple of days. Look forward to hearing your thoughts. Thanks for reading. Liv