okay see the truth of the matter is i have multiple headcanons for their first kiss because the truth is they had multiple first kisses
because there was Cas’ first kiss, the first time that was awkward and out of the blue and Cas sort of pecked Dean on the side of his mouth and then disappeared to leave Dean standing alone and confused and wanting in the middle of the room.
and it was a Thing They Never Spoke Of Again except Dean would find his eyes drifting down towards those lips more than he could help to the point where eventually there was Dean’s first kiss where after a particularly trying hunt where they barely scraped out of it alive Dean grabbed Cas’ face on both sides and kissed him and he was certain he’d never find his breath again. but Cas was so unsure of what to do that he stood still and didn’t kiss him back except to tentatively put his hand above the memory of a scar on Dean’s shoulder before Dean finally pushed away in embarassment and told Cas to forget about it
and then of course there was the first kiss where they were both involved in the kissing, which is, you know, the best way to kiss really. and it was a wholly unremarkable moment except how it wholly wasn’t, where it was a day they were really doing nothing of note at all, except sitting on a couch and where Cas leaned over to hand Dean a beer and Dean took it and sat it on the table. Cas looked at him confused until Dean’s hand was cupping his cheek tentatively before finding his eyes and there was no confusion at all about Dean’s intention. so Cas parted his lips and licked them and Dean’s gaze tracked the movements and it was so excruciatingly tantalising that Dean could not have stopped himself from diving in and licking those lips himself. and it was soft and slow at first until it entirely wasn’t and until their lips where red and their cheeks were red and Cas’ hair was a wild as ever, and they were happy.