Oh gosh, Dean. That part is the simplest thing to understand. Cas would've gotten it even as an angel, why don't you? "If our positions were reversed, would you look for me?"

The last thing in the world Castiel had wanted to see was that expression on Dean's face, and the last thing he wanted to hear was the anger and hurt in his voice. As long as he knew Dean and Sam were alive, that was enough for him. More than enough, and more than he deserved, really. Dean thinking Castiel was dead- it had been apt, and it was something Castiel was sure Dean could come to terms with.