Dean knew he was never the favourite son. Even from a young age. Dean did what he was told. He listened, took orders and followed where his John wanted him to go. He was less of a father and more of a captain but Dean still loved him. Even if they never really admitted it. He was loyal. He was everything his father wanted him to be, but it wasn't enough. He wasn't Sam. He could never be Sam. He could never be the favourite.
When Dean was younger and John left him alone the number one rule was always protect Sammy. Dean would do this automatically but it still stung. It wasn't 'Look after yourself and Sammy' it was just 'Protect Sammy'. He was a child who had lost his mother. Whose dad was never there. Who had to look after his younger brother when he was just a kid. He had to know how to shoot a gun perfectly. He wasn't allowed friends. He wasn't allowed to get close to others. He just wanted to be a normal kid, not worrying about his dad or his brother being killed or viciously mauled into pieces by creatures of nightmares.
Then the one time. When Dean had got tired being an adult and wanted to be a kid. He didn't realise that the Shtriga would come. The disappointment in his dad's eyes. God. That haunted him to this day. It pained him to know he had failed his dad and Sammy. It never crossed his mind that his dad was a little out of order. It never crossed his mind that this was wrong. That disappointment set his goal for the future. Protect Sammy at all costs and always be loyal. Another thing that never crossed his mind was how it was John's fault as well. Not the obvious but the fact that it slipped past him. He was meant to be hunting this thing. Not his son.
It hurt though. When Dean was dying. How his father never came. Never cared enough to look out for his son. Sure, he was probably off doing an important job but he was dying. He could have died. Then who would have looked after Sammy. Who would protect him?
Because that's what it was all about. Sammy was the favourite. He disobeyed orders and wanted to go to college but his father was still proud of him. Hell, he boasted to people about his younger son. Never about Dean. Dean was a tool. He was disposable. Sam was not.
That was why Dean was this way. Alcohol. Hunting. Girls. It filled a space in his heart that should have been filled in his childhood. The Winchester's pride themselves as being family and family being the most important thing but Dean wasn't sure why he followed it. Hell, most of the time it was that the Winchester's pride themselves as being able to protect Sam.
Dean loved Sam though and would do anything to protect so he happily gave up his father's love of himself to give it to Sam. He would give the world to Sam. Even if it meant giving up everything that he held dear. Even if it meant giving up his father's love for him.