If Dean was a thinking man, he might guess that it had something to do with wings, since that's probably where they'd attach if Cas had them. Well. Sort of. He's not really sure how it would work with a vessel and y'know what, he is not questioning it, because Cas just made the most amazing sound.

"Liked that, huh?" Time to figure out exactly what he did that made that happen. Was it the nails on his spine or the thumb at the base of his neck?