Dean wasn't the happily ever after kind of guy. He was never into the Meg Ryan/Tom Hanks kind of deal where he and Sam would realize that, despite the fact they owned rivaling bookstores, they could still fall in love (though that movie was definitely in the top 5 of Sam's all-time favourite love stories ever, something he would never admit to his brother). He never told Sam "I love you", and said that kissing was for homos and "I ain't no homo, Sam. You know what women call my penis? The Legend. My penis doesn't get called a legend by women if I'm a homo." There was only one reason why Sam knew that Dean even felt remotely sexual feelings towards him, and it was due to way too much whiskey and gin and Sam finally growing the balls to make the moves on his heavily drunk brother.
Though he had to admit, actually hearing Dean admit that he'd been wanting to fuck Sam since he went through that first horrible bought of puberty was pretty great.
No, Dean wasn't the fairytale sort of guy. There was no after-sex cuddling, no holding hands, no trips to Disneyland just so Sam could get one of those buttons that said "BIRTHDAY BOY!" as he ran around the park, making Dean go on rides like Space Mountain and buy him frozen bananas every time the temperature went up a degree. If it were a test and the subject was "Sam's boyfriend", Dean would get a solid D. Maybe a D+, if only because Dean let Sam top, despite all of his macho, older brother bravado. Dean wasn't Jessica, he wasn't soft and kind and loving like she was.
Hell, the only way Sam knew that Dean loved him as more than just his stupid, dorky, nerdy brother were the little things that he would do.
Dean would scribble all over Sam's research notes, little things like "hey bitch" or "suck my dick" or "O'Doyle rules" over and over and over again. To anyone else it would have been annoying, sure, but it was Dean marking, making sure Sam knew that he was always around, that he was always thinking about him so fuck all, Sam should too. And Sam did, always thought about the stupid smile Dean would no doubt have on his face as he lovingly etched out the Led Zepplin logo onto an article about a hell hound like it was his high school notebook.
Whenever a woman that was remotely attractive even so much as gave Dean "the look", he would snap his head over to Sam, look at him almost as if he was asking for approval. As if he knew that he was Sam's (which he was), and if Sam wasn't okay with it then neither was Dean. Well, that, and it was also probably because Dean was silently asking Sam if they were going to do it that night or if Sam was going to be too tired, but it was a romantic ideal nonetheless.
If life became shitty and Sam just wanted to quit and Jesus Christ, why were they the ones that had to fight all the time anyway?, Dean would stop, look at his brother, then put his hand onto his shoulder. It was heavy, calloused and rough like Sam knew it was, bearing all the metaphorical weight on his shoulders in a more physical sense. It said "Don't worry, Sammy, I'm here. I understand. And it'll be the end of the world before I stop being here for you, before I stop protecting you. I'll never, ever leave you, and that's the one promise I intend to keep until the day I die."
Every time they fell asleep in the same bed together and Sam had to move to the other bed, Dean would wake up in the middle of the night, his arms stretched out wide and hands thumping the mattress as if he was searching for his brother. Sam would watch with fascination at the pained look on Dean's face as his hands grabbed empty, cold sheets, would secretly smile at how suddenly he woke up, breathing hard and sweating cold bullets. It was that constant reminder of codependency that Sam got a rare view of, and he cherished that moment more than any other of the day.
Dean wasn't the perfect partner. Hell, he wasn't even the perfect brother. He was loud, obnoxious, lewd, and disgusting. But Sam had known perfect once, and as he sat in the passenger seat of the Impala, speeding down empty highways with Dean singing at the top of his lungs to an ACDC song, he figured that imperfect wasn't so bad.
Not if it meant Dean.