Based on Season 3, Episode 16 "No Rest for the Wicked"
Prompt: "If this is my last day on earth, I don't want it to be socially awkward" – Dean
Which for Dean meant no 'chick-flick moments' – no long heartfelt conversations about how they felt or how Sam was supposed to survive without him.
They had done the whole apocalypse style sex last night, before Sam had curled up into him and cried. And once he knew Sam was asleep, Dean had cried too: great big tears dripping down his cheeks and off the end of his nose as he pulled Sam tighter to him, knowing it would be for the last time.
But in the harsh light of day – a very ordinary day considering the circumstances – Dean had built his walls back up.
They were walls built on very shaky ground – one look from Sam, one declaration of feelings, and that wall would come tumbling down; crashing all around and breaking Dean apart.
He had made this decision for Sammy, just one year ago: he had sold his soul to the Devil and by the same evening had given Sam his heart.
And yes, he regretted that this choice had meant he left Sam behind – left Sam so much more lonely than he would have been before they started sleeping together – but when the alternative was Sam dying, there was no choice at all.
So as they drove Dean avoided Sam's eyes – knowing that leaving was killing his little brother anyway.