The Castiel theory wasn't something I'd considered. He seems to know what is going on with Dean, but he hasn't said anything. And it seems to me only bits and pieces are coming through to Dean and he hasn't really made sense of it all yet. It's just the past two episodes threw me a bit. Maybe Yellow Fever was a trigger. He looked unsettled in the last episode in the teacher's shop.

but when he does remember (if he hasn't already) it's gonna be great ;)