I think this moment in 15x15 is doing something bad to Dean's psyche. It was just... such an (underappreciated?) psychological blow.
First, Dean was obviously waiting up for Cas and Jack. It's adorable.
But notice: Cas was leaving without saying goodbye again. Here, Cas's panicking, his keys already in hand as he rushes out to find a (heroic) solution.
Hearthbreakingly, Dean actually asks where Cas is going twice in this scene. He can't hide the anxiety creeping onto his face when he asks.
They make small talk about their daily works. Cas twirls and pockets his keys, but there's a blink-and-you'll-miss-it moment where Dean tracks the movement of the keys with nervousness:
Dean forgets himself in the small talk, still laboring under the false promise, that the unethical plan will get his freedom (and everyone else's). There's a millisecond of happiness.
There's a big HUH??? He does the little head turn, the same one he will do in 15x18
Dean's anger rises, just a bit, alongside the panic.
The music swells and Dean, always clued in to when something is wrong, begins to get scared because Cas is scared. He flicks his eyes down to Cas in a panic, trying to understand what's happening, to him, to them.
There's a familiar sense of confusion and disbelief here, not wanting to believe the bad news Cas is delivering about leaving, about Jack, and visually and beats-wise, it's so soooo similar to how 15x18 goes down when Cas tells him the bad news about the deal and the goodbye.
I actually think this is the moment Dean's mental health sails right off a cliff. Of course it's going wrong. Of course good things don't happen to them. Of course.
