Creatures don't die in Purgatory, not really. They attack, and fight, and tear each other apart, cannibalize each other. And then they're reborn, less human, more corrupted every time.
Dean tries not to think that the same is true of him, that every time he was resurrected he's been less human, more of a simple killer. He tries not to think of it, because he believes that it's true.
It never occurs to Castiel to think that way. Instead of stripping him down to some primal creature, every death has left him more confused as to what, exactly, he is.