When Dean walked away and left his brother sleeping in that motel room, it went against every instinct he has.
Because Dean Winchester is his love for his brother. It's the first order his dad ever gave him, and it's the one that nested deep and grew until it consumed and transmuted everything else about him. No amount of years, no amount of torture and mistakes, no amount of knowing for certain that Sam doesn't have that same deep-rooted, unshakable drive will ever be enough to fire that out of him.
It's just…hasn't he always railed against destiny, fought against predetermination? Hasn't he always believed that the thing that makes monsters need killing, makes humans worth saving, is that to be human is to choose one road over another? A monster goes with its instincts, barrels ahead snarling and snapping at anything and everything it sees as a threat. Dean doesn't want to be a monster, not even for his brother, so he makes a choice.
He knows that if Sam is ever in trouble he will still come running as long as he has legs to run with…but right now Sam isn't in trouble. Sam's fine. In fact, Sam's better than fine: pulled from Hell, re-ensouled, rid of his nightmares and his visions and his hallucinations of the Devil. All he wants now is a normal life, and while Dean can't give that to him, he can make the choice to walk away and let him have it, and he will. He does, because for once in his life Dean realizes that he has the choice to make. Sam doesn't need him right now.
Somewhere, there's someone who does, someone who would walk through Hell for him, who has, someone who's laid waste to Heaven and opened up the Earth and tried to face the hordes of Purgatory alone, all for him. Dean isn't going to leave him there, not when he promised he'd take them both home. Not when every day that passes without him aches and throbs, gaping emptiness inside like a piece of Dean's soul is missing.
Hell, he thinks, it probably is.
He has no idea how to get back into Purgatory, but he thinks he knows where to start looking. The list of possibilities is long, but he bets most or all of them would just as soon kill him—or eat him—as look at him. It doesn't matter. Dean's made his choice, and he doesn't shy away from it. He doesn't look back.