Sam walked out that door so he'd never again have to pick up a gun with the intent to kill. He did it because he was Sam, and there was no reason he should get sucked into his father's revenge fantasy. Sam walked out that door so he could close that door. So he could have a life, a real life.
And he got that life. He got college. He loved college, with all the promises it offered. He loved it because he didn't feel out of place there. So many others were trying to bury unsavory pasts, too. Sure, most of their skeletons in the closet weren't quite nearly as literal, but Stanford was filled to the brim with lost people trying to figure themselves out.
He got friends. He'd never had friends, really. Dean was not his friend. Dean was his brother. Sometimes he acted like his father, and he always was the closest thing Sam had had to a friend. But biologically. He was his brother. Brothers didn't count as friends because they were brothers.
He got Jess. He'd never stayed in one place long enough to get close to any girl. Being a hunter was being alone, but Sam never meant to be a hunter. Sam was young. He didn't want to be alone. Jess was nice.
He got everything he ever wanted when he walked into college and left his family.
Yeah, Sam closed that door.
It's not like Sam missed hunting. Not when he had this life. But there were always moments that the youngest Winchester wanted to pick up a gun again, just to see if he still had it.
He wanted to know if he could shoot as well as he used to. He wanted to know if he could still beat his big brother in hand-to-hand combat. He wanted to know if he was still as silent as a cat when sneaking through private property at midnight. Or had college sucked these skills out of him?
These are things Sam was trained to do, whether he liked them or not. He harbored some pride for these talents that he'd cultivated, however impractical they were in normal life. Could he still hold his own in a hunt?
Not that Sam Winchester ever wanted to go back to a life of hunting. He didn't. Not ever.
Unfortunately, the key words there are hunting, life, and Winchester. Unfortunately, the door Sam closed wasn't anything special or fancy. The door Sam closed still had a crack underneath to let things in.
So the day comes much too early for his liking that Sam gets to confirm that, yes, he does indeed still have what it takes to make it as a hunter.
It's the day everything goes up in flames and the door burns down.