He never used to think about her hair.
The first year they met, she wasn't much more than a conquest. She was no-nonsense Detective Beckett with her no-nonsense haircut and when they had sex after his first near-death experience involving a shooter and a stranger's kitchen, he only felt a sense of triumph against the door of the precinct ladies room with her pants around her knees and his body draped over her back. He buried his face into her hair as he buried himself inside of her, smelling her cherry shampoo with every thrust.
When it was over and he was tucking himself back into his pants as she was checking her appearance in the mirror, he expected a sarcastic barb from her that dismissed him from his research duties as well as her life; to free him now that he had accomplished what he thought they both assumed to be his goal all along.
Instead, she ran a hand through her short hair and turned to unlock the door, tossing a "you coming, Castle?" back in his direction as she smirked her way out of the bathroom. Dumbfounded, he did what would soon become a pattern in his life: he followed her.
But then her mother's case happened and things got complicated and tense between the two of them. The summer went by, the book came out, and just as they were getting back to normal, his ex-girlfriend's bridesmaid was murdered. They both attended the small wedding the couple had after the case was closed – Beckett insisting that this was not a date, no matter how many times he showed her the invite that said "Richard Castle and guest" – and after she caught the bouquet and he teased her about it mercilessly, she ended up on her back on a table in a dark, unused hotel ballroom.
With her strong legs wrapped around his waist and her ridiculous heels digging into his backside, he toppled forward over her and came with his tongue in her mouth and his hands fisted in her hair and vaguely remembers noticing for the first time that she was growing it out.
She's guarded after their summer apart, less easily and harmlessly flirty with him and he can't quite figure her out. But after they get locked in that freezer together and she comes over that night because she can't get warm, all her layers of clothing somehow get discarded in his blanket fort anyway as they cling to each other, her face in the crook of his neck and leg hiked over his waist. Her hair is long enough that he has to pull it out of her messy bun to let it cascade down her back and tangle with his fingers, using his grip to pull her head back so that he can see her face when she cries out and falls apart around him.
He makes a joke afterwards about the irony of her alter ego's name and their current situation, trying to be lighthearted, and instead of laughing she chokes out something that sounds more like a sob, detangling herself from him as quickly as she can and throwing on her NYPD sweatpants and t-shirt, only stopping to pluck her underwear from the floor and shove it in her coat pockets.
He asks her what's wrong even though he knows – Josh – and when he realizes that she's not going to talk, offers to get her a cab, but she only shakes her head as she swipes her bra from the floor and heads towards the door. He never catches a final glimpse of her face behind the curtain of hair.
This time, after Beckett shows up at his door rain-soaked and just wanting him, he looks at nothing but her face, fascinated by the tear tracks etched onto her cheeks in contrast with the look of pure bliss as she lifts up and sinks back down on him, eyes shut and mouth open, wet hair hanging haphazardly around her head. He bends his knees up on one of her downstrokes and she groans at the change in angle, throwing her head back as Castle loosens the grip he had on her thighs, running his palms slowly up and down as she continues to move above him.
One of his hands flattens and skates up her chest, palming a breast as she opens her eyes to look down at him and he's taken aback for just a moment, suddenly realizing that this is the first time he's seen her face while they're doing this. A slow smile spreads like she can read his mind and she clenches around him, making his hands fly to her hips in surprise as he holds her to him and thrusts up roughly, a strangled noise leaving her throat. She slumps forward, breaking her fall with her hands on his chest as she gets sloppier with her hips, moving her hands to either side of him on the bed and simply grinding against him now as she grasps at his sheets with both hands.
He holds her tighter to him as he feels his orgasm starting to coil low in his spine, feeling as well as hearing the tips of her wet hair create a whispering friction against his chest. She's mumbling muffled nonsense, close, and he snakes a hand between them to rub against her and give a final thrust before she cries out, Castle following her not long after as she flutters around him.
His mind is so wiped that he might be mistaking the sound for the friction their rhythmic bodies are still creating but through the haze he thinks he hears I love you, I love you, I love you being whispered against his skin as she comes down.
He thinks of nothing but that hair for days.