Written for comment_fic for the prompt: Dean/Cas, the one promise I cannot keep.
Dean understands what it means to sacrifice for love, so he doesn't ask Cas any stupid questions. No "What was it like?" or "How are you handling the change?" since the answers were obvious: Crap and badly.
So there are never any conversations, and Cas just falls into their routine, hunting with Dean and Sam and staying in motor inns, trying to figure out his new fragile human-ness in the least pleasant way possible, by being just like the Winchesters. At night, he is exhausted, no matter how little they did that day, and he falls into the bed he shares with Dean without need for preliminaries or privacy; he is too tired at the end of the day for anything other than whiskey and sleep.
In the mornings, he wakes up to warmth and weight, Dean's larger body pressed close to him, the scent of both their sweat on the polyester sheets, and Cas feels it, waiting to see if this feeling means something or is just another things that his body can't help but notice, like the smell of manure on a farm or the sound of crickets in the morning. He shifts and Dean, barely waking, pulls him even closer, and Cas wants it even as it makes him want to run far away.
Once, before Sam wakes, when the sun had outlined the orange motel curtain with pink morning light, Dean wakes to Cas staring at him, with that puzzled look he's always had.
"I know it's hard," Dean says, cringing at how small a consolation his words must be. He has also sacrificed for love, and he knows that it isn't nearly as romantic as it sounds.
"It was my choice," Cas says.
Dean should nod and let it go, he should let Cas deal with it, the good and the bad, because really, Cas was going to have to figure it out on his own no matter how much Dean wishes he could fix it all for him.
Dean knows it, that there's nothing either of them can say that won't make it worse. But Dean is barely awake and more scared than he would like to admit, and so he does something stupid, does something totally unlike him. He presses his face into Cas' slender neck, kisses the delicate skin, and whispers, "Don't regret it. Promise me you'll never regret it."
Cas responds with a caress, a hand along Dean's jaw, a thumb's light press on Dean's lower lip. It is gentle, and meant as reassurance. But Dean notices anyway that it's not really an answer.