Dean's entire existence had been centered on Sammy. He knew Sammy before he was born, knew he would have a brother, and knew his mom would give him a Sammy as soon as he was old enough to take care of him. So he waited, and waited. And when he thought he was old enough he started praying to the angels Mommy said were watching over him to give him his Sammy because he was old enough to be big brother now.
Nine months later Mommy was in the hospital and Daddy was showing him a tiny wriggly thing that he loved purely and completely. It's not like he could feel anything else, its not like he needed to know Sammy; he already knew him.
When Mommy was burning on the ceiling, Dean was thinking about Sammy, tiny fragile Sammy with no one but him and Daddy left in the world to protect him now. That was one person less. He would just have to double for Mommy as well.
So Dean looked after Sammy the best he could. He learnt how to check Sammy's milk before feeding him, remembered to kiss boo boos after bandaging them like Daddy never did but he had to do for Sammy- just because he didn't have a Mommy didn't means he couldn't give Sammy one. He tucked Sammy in bed and tried to soothe his nightmares, tried to make up lies for him, protect him from all the evil things out there, protect him from a life which made his baby move around like he hated so much.
But Sammy was smart and found out, and Dad wanted him helping as well. Dean never quite forgave Dad for not just leaving Sammy at basic defense. Sammy didn't need to be involved in hunting; Sammy was his baby, just a baby. Sammy needed to be protected, not pushed to fight. But Dad wouldn't hear him. Dad was lost to him when Mom died, Dad was Sir now (he had been secretly trying to be Daddy to Sammy as well), and Sir didn't stand for disobedience. So Dean decided to protect Sammy as best as he could and teach him to be the best hunter ever. So Sammy could protect himself till Dean got to him if they were ever not together.
He was proud, incredibly proud when Sammy got his scholarship letter. Hurt and broken, but proud. He knew his Sammy. He knew how smart he was. He just wished he had done better, more, enough. He wondered if he should have tried harder to protect Sammy from Sir's words, to protect Sammy from everything that was making him leave them.
He drove Sammy to the bus station, and remained there long after Sammy had left (him). He wasn't quite sure what to do now.
He may have been Sir's soldier, but now that Sammy was gone, he didn't know what he was to do. He was Sammy's, always had been Sammy's.
But Sammy wasn't there anymore.
Dean tried to do what Sammy would have wanted him to, he carried on as best as he could and be as himself as he could. He still hunted, still protected the world in the only way he knew to, but he broke away from Si-Dad. (He had to remember to think of him as Dad). He did his own thing, and hid at the sidelines of Sammy's life, because what was he if not a big brother? How was he to do anything without knowing Sammy was safe? So if all his hunts included a get-to route with Stanford on it, then it wasn't anybody's business but his own.
He didn't want to pull Sammy out into the hunting world again. He didn't. He had to though, because he remembered Daddy, and for that memory of a father he had to enlist Sammy. He didn't want to do it without Sammy, because he couldn't do it without Sammy. He couldn't confirm the loss of one member of his family- all that he had left- without the only other member with him. He couldn't.
He was glad to have lingered for a bit when he dropped his baby brother off. So very very glad. He had had to pull his Sammy out of the fire for the second time, and he would keep doing it, over and over again. He didn't know life without Sammy. He wasn't anything to anyone except to Sammy. He was conditioned for his brother, trained to his every need. He wasn't anything except Sammy's.