(good) bye and (good) night
Cas doesn't like goodbyes, and not just because of the feeling they give him.
It's not like you have the chance to say bad-bye or awful-bye or if-you-leave-it's-gonna-break-me-bye. No, byes are always supposed to be good, even though they so rarely are.
It's the same thing with nights. You can't wish someone a bad-night, or a by-the-time-you-wake-up-I'll-be-gone-night.
He hates byes, and he hates nights. They just never felt good to him. Byes are lonely and heartbreaking, and nights are cold and dark.
It was night when Dean first broke up with him, and it was bye he said to him when he left the second time. And even now that he's back, the byes and nights still linger in the background, making him shiver every time Dean doesn't make it home on time.
But they are supposed to be good. And so he sucks it up and kisses him goodnight and goodbye, when all he wants is to kiss him I-can't-lose-you-night and please-don't-leave-again-bye.