Dean spends a lot of time angry lately. Okay, yes, he's spent a lot of his life in general angry, but lately it's bothering him more. Go figure.

"Oh for--Orphans, Cas? You bring up the damn orphans?" Hello, sore point.

Sure, Castiel probably means to imply that even the worst decisions come from a point of goodness, but this is Dean. He's good at missing the point, so all he sees is the old "good job, Dean, you screwed that one up" barb.