It was his moment in Gethsemane. He didn't want the responsibility, but now that he knows that he is the one to bring about the problems in the first place, he won't ever shirk it.

That's how I see Dean as well. It's interesting you brought up Gesthsemane, while ivy03 was saying how Castiel's a little bit of a father figure. Because Dean had a Gesthemane in season 2--at John's grave in the AU world. Where he pretty much said, only less broken, a similar idea--why do I have to be the hero, why me, I don't want this.

But then Dean steps up and does what he has to do.