I

You told me once, in one of those motel rooms. You told me you were afraid you might kill yourself if you ever saw what you did to Heaven.
Cas has been looking at him since he got in the car, and his gaze remains steady even now. I did say that.
Dean swallows with a little difficulty. We should have addressed that better.

Castiel struggles with the effects of fading grace, Dean struggles with the effects of the Mark of Cain, and sometimes Castiel is the only one who can understand what Dean is going through.