Keep this in mind - all I know is whatever has been aired (first run and reruns) from the werewolf episode (<"Heart"?) on, plus up to "Scarecrow" from the first season, and all the stuff I've seen on my friendslist. So there is a lot I don't know.
But this is how I'm seeing them right now (and, yeah. Oh, Dean works too,too well.) And this is stuff I've thought since, well, the first episode I saw, and I know it's been confirmed in this latest one.
Dean doesn't have a life. He's never had a life, not since he was four years old. At four, everything was taken away from him - Mommy, Daddy (too busy hunting), stability, home. Security. And in child logic, the way to keep whatever he had safe (because clearly, he did something wrong to make it all go away, even though he didn't) was to be a good boy and do what Daddy tells him. And what does Daddy tell him? Learn weapons, hunt demons, find who killed Mommy - all of that. But mostly, he was told to watch Sam. He carried Sammy out of the burning house, and he's taken care of Sam ever since. It had to be a constant refrain. "Watch the baby." "Take care of Sammy." "Look out for your brother." And he did, and he was good at it. He was probably changing diapers at the age of five. At nine, he took care of his brother for weeks. This is who Dean is.
As mecurtin said, he's never made a choice in his life. And he hasn't. I'm thinking about "Scarecrow." Daddy sent them there, so Dean went. Even when Sam left, he kept on going because Dad told him to. Sam could choose to leave, Sam could choose to return. Dean could not. From what I gather, this entire second season he's been keeping a secret from Sam (because his father told him to) but not really doing anything to help out - just avoiding the day he might have to kill his brother. And there it is - his two basic motivations in direct conflict.
Dad tells him to go someplace,he goes. Sam and his visions say go someplace, he goes. ASH tells him to go someplace, he goes.
This deal with Death is maybe the first tiime he made a choice on his own, and even then, it was takikng care of the baby, using the only coin he had.
And people are saying it's the last season redux, but it's NOT. Because now it's on Sam to save *him*, and Sam is NOT passive. Sam doesn't just do as he's told or wait for directions, and no one tells him something is impossible. And he will do what he has to do, which is why he killed Jake (and why he's the one who shot Madison.) He's going to seek out ways to save Dean. He's going to take real chances on his own to do it. He's not going to just angst in the night.
On the other hand, if he loses the fight, if Dean goes to Hell, or even if he dies and goes someplace else, Sam will survive. He's strong enough. Will he hunt demons? Maybe. Or maybe he'll find his way back to law school. He loves Dean, would die for Dean, would kill for Dean, but Sam (maybe thanks to Dean, who raised him) will survive.