Sam knows there's something wrong when he's old enough to realize what idolizing and hero worship is. He knew Dean saw their father that way, always bragged that he was some kind of vigilante out there saving lives. Sam never saw it quite like that, it was more so a drill sergeant who only cared about the mission, but he could share in the sentiment. He thought at first that it was because Dean tried to emulate their father, pick up his mannerisms, wear his hand me downs like a uniform when he was finally big enough to fit into some of his clothes. But there was something so different about Dean versus their father.
Dean, while buying into the hunter persona, still had bits of humanity about him. He still looked at him with soft and kind eyes, tended to him when he needed him most, and not just because John was always barking orders to 'keep an eye on Sammy'. He could tell that Dean genuinely cared, that he prized something above slaying the monster and getting the girl, as much as he would try to deny it.
His big brother didn't see him as a grunt. He coddled him and offered him emotional support (to the best of his ability) as Sam went through the motions of growing pains, physically and through his psyche as he gained a deeper understanding of there being something greater and bigger outside of the hunting world. He needed something more than he could get from his brother, and that's when he realized that the hero worship was gone.
It was replaced with something else, something that made his stomach twist in knots, like he knew deep down that he shouldn't be looking at his brother like that. And it was only when he noticed that Dean was looking at him the same way that he realized it was trouble.
Glances lingered, the air seemed to prickle with static, breaths caught and grew shorter. It was a game of chicken and deep down he knew that Dean would be the first to cave, so he did his best to ensure that never happened. It couldn't happen, it would ruin everything that they had shared together. It would tear the family apart, John would either kill one or the both of them, or even worse, lose all respect for them. Sam couldn't stomach the idea of Dean losing his hero, so he made his decision early. He'd leave, and in the process remove the poison from the well.
