I was just think that, what if, in maybe season 5, Cas picked up on how a man's wife or girlfriend kiss them goodbye as a send off to a war usually. And maybe Cas decided to kiss Dean goodbye one time and, Dean made a joke about it and maybe said A pat on the back would've done fine, and that's all that comes of it because kripke thought it would be funny.

Then, maybe, skip ahead to season 12 or 13 and it still had never been brought up because it was just a joke, a bit. And maybe Dean and Cas are alone in the bunker kitchen, sharing a beer and Dean says, I'm thinking about 2010. And Cas says Why and Dean says Remember that time you kissed me goodbye? and Cas says Yes that was very embarrassing for me, and Cas thinks of leaving because he's not ready for this, he'll never be ready for this he thinks. And theres a bit of silence before Dean says I think about that a lot. And Dean is not looking at Cas, he's just staring at the counter, and Cas doesn't say anything. They finish their beers and Cas says Me too. Dean goes to bed and Cas goes for a drive and maybe he cries a little and maybe Dean does too because they know they won't bring it up again, at least, not for a long time.