Dean watching Cas talk about something overcome with how smitten he is, fully not processing anything Cas is saying just going mmhm yeah yes totally, his head repeating cas kissing cas mouth mouth pink cas love him kiss Cas. So he doesn't even really notice himself leaning in and doing it until Cas pulls away and goes "um. ?" and Dean's brain clicks back on at 100 miles per hour. He says "I um. I didn't..." he looks around and sees Sam's bug eyed surprised face bc he was there this whole time bc it was just a normal conversation. Dean says "I just gotta. Um. I'll see you around." And fucking legs it.
Cas and Sam are left in awkward, confused silence. Cas puts his hand up to his mouth and, after a beat, asks "is the formation of rock pools really so stimulating?" (Bc that's what he'd been talking about) and Sam's halfway through saying "I have no clue, man," when Dean bursts back into the room, grabs Cas’s face and kisses him thoroughly because, fuck it, if he's going to have to deal with how awkward he's just made things he may as well have a kiss he can remember out of it.
Obviously, Cas gives as good as he gets, and Sam's like (amused) "okay I guess we'll talk later" as he makes his escape. Dean, with his hand already making its way up under Cas’s shirt, calls "much later. Next year, Sammy" (Cas kisses his neck) "next DECADE."