The way Cas is eying Dean while sucking on the ice is probably telling as to how badly he'd like to make a move right now.

Still, it doesn't seem appropriate. Dean's obviously upset about Sam leaving, and Cas has made himself too concerned with the Leviathan and giving Dean space to make an attempt at puzzling out all the sources of Dean's depression. He should make more of an effort; ignore the Leviathan problem, at least for now, and try to put Dean at ease.