There was a time, fairly brief in the grand scheme of things, when Sam and Dean each decided they were too old for brotherly affection, and kind of ignored each other. They were teenagers and naturally absorbed in their own worlds of girls and school, respectively. Dean might grunt when Sam came home from school, or Sam would mutter something random and nerdy about the nutritional content of reheated macaroni and cheese over dinner, but for the most part they stayed out of each other's way. In brief flashes of guilt one or the other might have wondered if maybe it was wrong, this adolescent phase, a betrayal of so much love and trust. And maybe it was.
These were back in the days when Dad still only paid for one room with two beds, and despite their man-child sizes, and oh so mature insistence on personal space, most of the time, when Dad was home, they'd share the other full bed, farthest from the door, and they'd fall asleep, Sam's hip pressed against Dean's side, and their feet wrapped around each other's for warmth, and in moments like these, they'd be brothers again and the guilt would fade away. Nothing could go wrong in a world where you fell asleep to the soft hshh hshh of familiar breathing, and the feel of the warmth and remembered shape of your childhood pressed against your back, stealing space and sharing love at night.
The phase passed eventually. Actually, it lasted until Stanford. Dean sometimes wondered, those four miserable years, if, maybe, if he'd paid more attention to Sam, Sam wouldn't have left. Somehow, maybe, those feelings at night (that they were all right after all) had been phony, invented by his "ego" (Sam would say) to sooth the guilt of his "id" (Sam would say), and that Sam had really felt neglected. But there was nothing for it by the time Dean thought of these things. Sam was gone, and was probably never coming back.
When Sam did come back, Dean was rougher around the edges – four years hunting without the one thing that had kept you human your whole life will do that to you – and not quite sure if boundaries Sam had erected when they were teenagers (Dean. I swear if you touch me again-) were more sincere now than they had been. A whop over the head occasionally all in good fun (and Sam's reaction carefully, and unobtrusively observed), or a bump on the shoulder was all they got for a while. But with time and presence (and sure, all the stress about Sam's visions and destiny helped like a pressure cooker helps make a mean stew) they slipped back into old ways like into worn jeans. They didn't have any reason share beds any more, what with Dad being gone and all. But they bumped and shoved and rubbed and pushed enough during the day to make up for it. And sometimes, when Sam still felt so empty his heart should implode from the strain, or when Dean just needed some extra reassurance that Sam was here, really here, at least for now, one brother would slip into the other's bed (it was really too big for one person anyway) and curl up on the edge, and listen to his breathing; and like moons and planets they'd slide together in their sleep (or maybe it was just that the stupid motel beds were curved inward so gravity naturally pulled sleepers toward the center. Yeah, that must've been it) and Sam would tuck his hip against Dean's side, and Dean would shove his toes under Sam's feet and they'd sleep and rest and breathe in certainty and hope.