Because of her eyes and her giggle and the relief on his face.
Disclaimer: It's not mine, I could never claim to own such brilliance.

Seeing the difference.

He watches her leave, still stunned by the look on her face, frozen into immobility by the terror that bathed her eyes, the raw emotion that flickered as she lay her heart bare, before relief and hope both morphed into pure joy that just...bubbled right up out of her.

He needed to kiss it.

Just touch a little bit of it, share in it with her because they truly are amazing and they should revel in it, scream it aloud and enjoy it, and then she's close, so close and so ready in forgiveness that he can't quite believe his luck.

And then she's dancing away and scrunching her face up, and...miming boobs in front of his eyes and gesturing madly and a little disgusted, and so ridiculously cute it hurts his heart.

Who is this woman?

So, he stands and watches her leave, hears her pad into her bedroom, the soft movement of sheets and pillows that is almost too quiet to recognize save for the fact he's heard them every night this week, every night since his family flew out of the country, and his lips lift the slightest amount because she walked away but she didn't ask him to leave.

And this is what? Their first real test?

Dammit, he's not going home, it took him four years to get past the subtext and the layers and the damn front door. Four years to get to a point where they can stand here and talk and have it all be ok, painful and scary, but still ok.

Four years to get into her bedroom, and he rubs his face, dispels a bit of his own unease as he works out what to do.

Because it's not about getting back in her bed, or her pants, or under her nightshirt - that still takes him by surprise - it hasn't been just about sex with her for a very long time. It's not even about breaking down her defenses.

This Beckett has no wall!

This Beckett, Kate, has no hard armour and she's open and vulnerable and so young in her ability to love him, the magnitude of it smothering her face and weeping from her eyes, the quiver of her lips, loving him so intently...and he hurt her...

She's so different to what he expected.

He kicks off his shoes and tosses his jacket onto her couch, not presuming more than that because it's not about getting naked and proving things with sex.

It's about the look on her face.

The way her eyes glistened with trapped tears when she worried they would implode.

It's about the way she said the word -

Implode.

As if she could taste the pain of it already.

And he needs to make it right, make it better, he needs her to know how much it means when she shows him every fractured facet of her beautiful personality, her dented soul.

She's so different to what he expected.

He circles her kitchen, reaching for a cup with ease and it tenses his entire chest, his heart contracting, pounding so hard he has to lean into the counter and brace himself. It crashes over him like thunder as he realizes just how far they have come in such a short space of time.

He knows his way around her kitchen, he knows where the spoons are and which shelf holds the chocolate that will make her elicit that deep wavering groan of pleasure when he slides it over her tongue.

He takes a second, another breath, giving her space to settle and him a moment to collect himself. It just continues to catch him off guard, that she's in it with him, that they are doing it together, making mistakes and learning and screwing up and yet...The need to see her, to fix it, to be near her and love her is still burning bright in the background.

He unbuttons his shirt, slipping the top two free so he can run his fingers around his neck, picking up the mug and turning back towards the bedroom.

Drawn back to Kate.

She hasn't closed her bedroom door, just pushed it halfway shut and hope ripples through him again. He palms the wood slowly, fingers over the ornate pattern in the center of her door before he peers around it.

She's pulled the sheet back and her toes just dip below the folded line of material. Her fingers play across her knees, tracing the hem of her grey nightshirt. Smoothing it down until he takes a tentative step into the room and her head lifts.

Eyes still tender, still aching and soft. Still pained.

And he crosses the room to sit diagonally from her at the foot of the bed. "You're different." He says, folding a knee beneath him to turn towards her, head tilting as he puzzles her out.

A mystery he was never going to solve, he was not wrong. But maybe a little understanding...

"I'm pissed off." She says quietly, almost shrugging as if he should know, pulling the shirt lower and drawing her knees up under her chin. Her eyes lift and fall, skirting the lone mug resting against his thigh, back up to his face and away again.

"That's not what I mean." Castle stands and moves around the bed, coming to rest at her hip, needing to be close even if she pushes him away again.

But he's on the wrong side of the bed now and when he sits down, he has to squash her, shuffle her over and Kate huffs with annoyance, her lips scrunching and maybe, maybe, just a trace of humor mixing in.

Castle gives her a weak smile and holds out the cup, her eyes dropping to it warily even as her hands rise to take it from him.

She frowns, "If you think bringing me coffee in bed is going to make up for..."

He watches her glance down, eyes widening in shock before she looks back at him, confused and vulnerable and even this, this tiny gesture, hurts her or scares her, he's not sure but her eyes are wide with it, sparkling and exposed, she leaves herself open to him.

And it crashes over him again, that even though she hasn't said the words and he has and it lingers between them everyday, how much she is investing in them, in their time together and in being enough...how much she is investing in him.

His hand lifts, drawn to the warmth and worry of her face, sliding his palm over her skin to cup her cheek, his thumb tracing across the bone as the tips of his nails catch in her messy ponytail.

"It's a promise." He says quietly.

Her eyes flick down, but she doesn't pull away or shove him off. "It's an empty cup." She looks back up at him, a little less fractured than she had been earlier. When he had been talking of maybes and what ifs in her living room. Trying to reassure her until his faith in them being amazing - because they are - put that light back in her eyes, and she was happily threatening to shoot him.

"You're different to what I expected." He says softly by way of explanation. "Open and willing," his other hand coming up between them, unable to resist the curve of her cheek as it lifts, the arch of her eyebrow when she silently questions where he's going with this.

He touches at her lips, fingertips brushing a shallow dip as he smiles and traces her skin, "You're soft and eager." The depth of her eyes, eyes that still shimmer, pulling him in. "You're tender and vulnerable and I'm sorry I hurt you."

Castle leans closer, her lashes beating as rapidly as his heart, "And it's not an empty cup." He stops when his forehead rests gently against hers, breathing the words into her skin, "It's a promise that I will bring you coffee everyday, to see you smile the way you did earlier."

His fingers press the words into the flesh of her lips, the edge of her jaw, "The way you do when you leave for work and I get to kiss you goodbye."

Their noses brush, lips parting and sharing breath but neither moving closer. "The way you do when you joke about me wanting you for four years and I know every word of it is true." He shudders as it takes his breath away, forcing himself to breathe before he passes out in her lap.

Because he is just as vulnerable and open and soft and eager and he tries to show her and let her in on the secret of how she stole his heart.

"The way you smile Kate." He takes another deep steady breath as her fingers curl around his wrist. "Just you."

Her eyes close as she tips into him, the empty cup discarded in her bed and both hands clinging to his wrists and holding him to her. He feels her lips part, the shallow exhale of her breath over his skin, her thumb tracing the outside line of his hand, curling through the palm.

She sighs and he can hear everything he needs to in that breath, "We just decided to be exclusive with very little effort, I think we've got this." Because he honestly believes they do. "And I am sorry if I made you feel like we didn't, that us not seeing other people even needed to be brought up."

His fingers squeeze, just enough to get her eyes to open, so he can see her beautiful face properly when he presses her back, "Because aside from the fact you threatened me with your gun." He waits and she smiles, eyes narrowing in warning, "Aside from that, you're it for me and I am so so..."

Her finger presses against his lip, silencing him when it slips between them to touch at the penitent pout of his mouth. Then she breathes out, pulls him in and replaces her fingers with her own lips, kissing him softly as he caresses her cheeks.

She's so different to what he expected, making him want more, need more, strive to be better.

And together, they are amazing.