"Dude, you've never been an open book. Your a locked book, in a locked box, buried in a locked crypt." But Dean had laid the opportunity for Sam to ask questions out, and he wasn't going to give that up so easily. "You didn't answer me back at that house... Why were you praying to Cas? You said it yourself he's not been right since he got out of Purgatory, so why were you praying to him?"
He kind of got it though. No matter what they went through, Cas was still technically family.
"Why didn't you just ask me if you were worried?" He would've lied, but that was beside the point.