Dean frowns, not quite confused, but... okay, a little confused. So Cas is from his future now? Dean still isn't as up to speed as Cas is? Yeah, he's doing a lot better than he was, but that's still annoying.

"Ssh," he says, and he touches Cas's cheek. "It won't be for long. Never is."

That's big talk coming from Dean, who's almost always afraid the next time Cas walks out is going to be the last one, but Cas looks like he needs the reassurance, and Dean wants to give it. He wants to believe in it because he might never have come back from this one.