Sam: Dean, look, I know you think that Cas is gone —
Dean: It’s because he is.
Sam: He’s not! He’s in there somewhere, Dean. I know it.
Dean: No, you don’t.
'Dean simply cannot wrap his head around a Cas who would make such grievous mistakes, so he writes the angel off completely. And it makes sense, because this is what happens when you disappoint someone who doesn’t have relationships but “applications for sainthood”. When you fail him, he discards you like the idea you are.'
'This episode is mostly to advance the overarching Leviathan storyline, but there is that bit at the end where Dean finally admits to killing Amy – which is another indication that he listened to what Sam said last episode and is trying to talk about his feelings. Of course, it backfires, and Sam leaves.
That’s an important moment for Dean – just because he’s willing to admit his guilt doesn’t mean that others won’t be mad at him for the wrongs he committed. And nor it doesn’t absolve him of his wrongdoing. Dean realizes that he can’t just let Sam go now; he’s still got to put in the work to make things right.
Keep this in mind for when Dean gives Cas the “nobody cares that you’re broken” speech in “Survival of the Fittest”.’