New headcanon: one time while Sam was out Cas got sick (while human) and just wanted to sleep near/on Dean for comfort. Dean reluctantly accepted but in actuality he wanted to make Cas feel comfortable, and after a while they fell asleep like the picture. When they both woke up they claim to not remember or scoffed it off like it was no big deal, but Dean thinks about it if he's sad to cheer himself up
Dean, I… When I was… bad… and I had all those things – the… the leviathans… writhing inside me… I caused a lot of suffering on earth, but I devastated Heaven. I vaporized thousands of my own kind, and I – I – I can’t go back.