Cas has no idea what changed but hell if he's looking at this particular gift horse in the mouth.

No matter how tightly Cas wraps his arms around Dean he can't get their bodies close enough. He picks up tips, techniques, what to do with his lips and tongue by what Dean's doing to him and puts all of it to good use. The room gets even hotter and Cas is panting for air wherever he can get it; his fingers curve and he rakes them down Dean's back because that seems like the right thing to do.