Remember when Cas said, “I always come when you call”?
Can you imagine how hard it was for him to ignore all of Dean’s prayers? To hear him every single night and not go to him? Because he’s always been there for Dean. He always went to Dean when he called, even if Dean couldn’t see him.
But he has to stay away because he can’t protect Dean by being with him. He’ll only make things worse, and he knew that this was the best thing he could do for Dean.
But now, standing before the man he raised from Perdition, the Righteous Man, the man that taught him what it was to be human, he can’t say no. He has to go with Dean, because he’ll always come when he’s called.
And he’s being called home.