It's my job
As their father approached him, placing a reassuring hand on his shoulder, Dean took in a deep breath then exhaled it, releasing the tension he had been holding for what seemed like centuries, since before Sam had died, since their father had died. The look of pride on his father's face needed no words. The tender, paternal squeeze on his shoulder spoke volumes to Dean. It was that sense of approval which confirmed that he had done the right thing. He had done his job. He had saved Sam the way that his father would have wanted him to. His life, for his brother's? No contest.
When Dean watched their dad look over at Sam, the subtle nod and smile told him that he was just as happy to see Sam alive and intact as Dean was. Their reunion didn't have the sadness and regret that seeing their mom at their home in Lawrence had. She had uttered their names and apologized to Sam, then faced the poltergeist with the same courage she had faced the demon with at Sam's crib. Dean knew that he had avenged her as well as his father. He would always carry the pain of those losses as Sam would too, but he knew that it was his responsibility to make sure that Sam wouldn't be sacrificed, at all costs. No one had to tell him that. He had known it the moment he had held Sam in his arms that night.
Bobby's tearful tirade about how little he valued his life wasn't altogether true. He hadn't the strength to argue with him about it. It may have seemed that Dean was devaluing his life, but Dean knew it was more than that. He had told Bobby that he had wanted his life to have meaning, and without Sam, there would be none. He couldn't let Sam die. Like he had told Bobby. He just couldn't. His world without Sam in it was desolation. If he had let Sam stay dead, Dean would have ended his life shortly thereafter, wasting it in a way that their father would never have been able to forgive. Trading it and his soul was the best and only way he could think of to give his life, whatever time he had left of it, meaning. He wasn't supposed to live after the crash. Their dad had given him that speech, had entrusted him with the promise to save Sam, then had given up his life because he had known that Dean would make the right choice, that his life was meant to be given up, if necessary, to save Sam. He had known that Dean would get the job done. They had been in his last words to him, "You took care of Sam. You took care of me. You did that. And you didn't complain. Not once." He had to take care of Sam. That sacrifice gave him the kind of meaning he wanted. He would cherish every moment left to him with Sam by his side. Sam's flesh and blood existence was more than enough proof that it had all been worth it.
And like he'd told Sam...it was his job.