When Castiel first laid a hand on you in Hell, he was lost!
Deep down Dean knows it's true. After all, it's always been his fault.
His father's death was his fault. Sam's lost soul was his fault. And Cas…
He had tried to blame him. Really, he had. Hell, he had tried to hate him. But Dean just… he couldn't, not really. Castiel meant too much to him. Castiel believed in him. Castiel had changed for him. He had developed his own opinions, his own reasoning, his own ideas. And one of those ideas, was that Deancould save everyone. And Dean hadn't disappointed him, not at first, anyway.
But when Castiel failed, Dean knew if he had just tried harder, he would have been able to stop him. But he had been too slow, too sloppy.
In the end, that too was his fault.