Who do you think fell first, Sam or Dean? As always I have a million versions but the one I feel is most committed to is Sam in this version:
Sam is four years younger, so Dean's already well into puberty when Sam is just starting it. Sam is suddenly overwhelmed by hormones and getting random boners and having wet dreams, all while his stupidly pretty big brother is right there all the damn time. Most kids get crushes on older kids who seem (and usually are) so unobtainable, and Sam's fucked up because sure, he notices cute girls and is suddenly very interested in tits and glossy lips but he also can't stop seeing Dean in a whole new way.
Sam's still too young for Dean to be attracted to him, he's still firmly in the box of little brother/child in Dean's mind. He makes things so much worse for Sam by teasing him about all the awkward body stuff he's going through because that's what big brothers do. He doesn't have any clue how much he's killing Sam just by existing, the way Sam is mesmerised by the golden glow of Dean's skin in sunlight, the shift of his muscles, the shape of his ass, the cute dusting of freckles that Sam knows spreads across his whole body and he wants to taste with his tongue, the red of his lips when he's been chewing them - fuck, the fucking oral fixation he has chewing on everything and anything. Just everything about him was made in a lab to drive Sam insane.
He doesn't have any idea how hard he's making it for Sam. They have to sleep in the same bed. Dean gets morning wood and Sam wants to take care of it for him. Sometimes they end up tangled together and Dean just laughs off the awkwardness of them both being hard. Sometimes Sam suffers the humiliating experience of Dean either gently or not gently pushing him away when Sam ends up humping him in his sleep. Sam is terrified he's gonna moan Dean's name while he's dreaming because he dreams of him all the time.
Sam also forces himself to stay awake longer than Dean just to look at him while he jerks off, and sometimes he's brave enough to pull the covers down and touch him - just on his arms, his chest, his back, nothing too- okay fine sometimes his ass and his thighs. Of course he fantasises the whole time. Dean decided showing Sam skin mags was a necessary part of The Talk and it was all straight porn but he knows gay sex is a thing, and he can't. Stop. Thinking. About. It. He wants to fuck Dean. He wants to be fucked by Dean. He wants everything.
And they're in such close quarters all the time so sometimes he walks in on Dean watching porn and jerking off and Dean squawks at him to fuck off. Dean talks about his girls and the things they do and Sam seethes with jealousy. Sometimes Dean jerks off right next to him in bed when he thinks Sam is sleeping and Sam memorises all the choked back sounds he makes and if he is feeling brave he peeks.
And over time he gets other crushes and realises they don't feel half as intense as the way he feels about Dean. He's not only jealous of Dean for being cool and handsome and older and so cocksure and confident, but he's jealous of everyone who gets Dean's attention. He wants it all to himself. He wants Dean all to himself.
It takes years for Dean to notice him in the same way. A few growth spurts and a lot of muscle packed on from Dad's ruthless PT later, and suddenly Sam catches Dean looking. Finally.
Of course I also have a more depraved one with Dean being obsessive and weird about his cute little brother before Sam even really understands what sex is, but this is my favourite one. I love pining.
