When Sam first notices Dean, and I mean as a guy not as y'know Dean, she's 16. It's not anything flashy, no big sudden burst of oh wow Dean is actually really attractive wow who knew? It's more of a soft, quiet thing. It's just after a hunt and Sam had been crouched in front of Dean wiping blood off his forehead and maybe it was the way the crappy, motel lighting made Dean's skin glow, or maybe it was how his banged up jacket hugged his shoulder's just right… Or maybe it was nothing at all. Maybe Sam had always felt this way but now the veil had been lifted and Dean was no longer 'Dean her brother,' but 'Dean the attractive man.'
So safe to say she was pretty freaked out. But once she calms down it's actually not that surprising. Dean has always been the most important thing in her life, the rock she can lean on and depend on in their little world of hunts and motels and Dad never being there (not really) and driving-driving-driving. So yeah, once Sam thinks about it's not surprising that she has a crush on her brother. And it is just a crush. A stupid childish crush and she'll get over it soon enough because brothers and sisters they don't do stuff like that. And Sam's life is messed up enough without her confessing to her elder brother that she might, possibly, be in love with him. (but it's only a crush right?)
It's a blessing in disguise when her acceptance letter to Stanford arrives. And there's a fight and Sam leaves and when she does she doesn't look back because if she does and she sees Dean (what crying? Pleading her with his eyes not to leave him, not to break their family apart again? (But Sam can't remember a time when their family wasn't broken)) she won't be able to leave. She'll turn back and tear up the letter and never leave Dean but she has to.
So she does. They don't see each other for years and Sam can almost forget her feelings for Dean. She meets Jared and maybe falls in love with him, could see herself making a home, a life, with him (but his skin is too soft and too smooth. It doesn't have any of Dean's freckles and scars and it doesn't seem real because of it. It feels fake, artificial).
Then Dean comes back and Dad's missing and she's hunting again and Jared's dead (stabbed in the stomach and burned on the ceiling. A macabre parody of her mother's death not that she knows it) and suddenly it's her and Dean again. And she's drowning, drowning in grief (oh god Jared I'm so sorry if we'd of never gotten together this wouldn't of happened to you) and desire (Dean's right there and he smells just as good as he did before and he's got new freckles on his cheeks) and disgust (what's wrong with you? Not only is he your brother but your fiancé was killed less then a month ago you sick selfish fuck).
The first time they kiss is…almost an understatement really. It's simple (but it's not it never is) Sam died and Dean sold his soul to bring her back (oh but he shouldn't of, Dean is wonderful and kind and good and Sam is twisted and wrong and if Dean knew the truth he would be glad to be shot of her) The first time they kiss Sam's just woken up from a nightmare and Dean is right next to her stroking her hair and talking to her gently (and isn't Dean always there whenever she needs him except when she ran away to Stanford) and Sam buried her face in his chest and sobbed like a child and Dean hummed a little tune, the same one he'd always hummed (their mother used to sing it Dean hums it so that he won't forget and when he does he feels a little closer to her) and Sam had pulled back to look at him (to what? Apologise? Go back to sleep? Tell him she was alright? Sam can't remember) and suddenly they're kissing. Dean's lips are plump but chapped and the angles wrong and their noses keep bumping and Sam can taste the crappy chinese food they'd eaten as well as the beer Dean had drunk. It was sloppy and fumbling and perfect.
Until they had to stop. And when they did Sam had to hold back vomit because oh my god you kissed him and he's your brother and he's going to hate you and ohgodohgodohgod. When she tried to run Dean wrapped his arms around her pulling her close and crooning in her ear. 'It's OK Sammy it's OK.' Over and over. But it's not OK. Dean is her brother and brothers and sisters don't do stuff like this and why doesn't Dean hate her. Then she starts to actually pay attention to Dean's mutterings. 'I'm so sorry Sammy, I swear I won't touch you like that again. I didn't mean to. Oh god Sammy I'm so sorry.'
But- What- …Sorry? Dean's sorry? Then it clicked. Dean had held her, had kissed her back. Sam froze and stared at Dean, for an eternity they stared at each other. Then they were kissing again. But this time…it was better. Because Sam was sucking on Dean's tongue and Dean's hands were dragging over her skin (his calluses felt so nice on her skin, little pinpricks of extra sensation) and her hands were rubbing against his skin (scarred and marked and imperfect. Real and there, nothing fake or artificial) And Sam lost herself in the salt of Dean's skin and the heat of his body and when she came, she screamed out Dean's name, Dean sobbing hers into her neck.
It was only a week later that Dean was dragged into Hell and Sam was left alone with only a hickey on her neck and the memory of Dean's touches.