Dean went to hell for Sam but loosing Dean was hell for Sam, too. And Dean couldn´t stand living a day without him and brought him back. Sam had four months without Dean. He was so lost and hit rock bottom after Dean went to hell. He was and is still feeling guilty for not saving his brother and the hunting was the only thing that kept him alive, I believe. But with no Dean on his side he had to find a way to make it work without him.

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Their last scene in the car when he tries to understand Sam and apologize that he was to hard on him is exactly what Sam needs and not one who blames him for what he is or what he can do.