Honestly, there's a part of Castiel that's impressed with how well Jack has kept it together in the face of his constant rejections. Try as he might to be gentle about it, rejections are exactly what they are, and he's never been able to deny it to himself. Now, though, he isn't feeling particularly charitable, even if he doesn't immediately snap back in the face of Jack's accusation.

He lets himself chew on it.

"Of course I'm afraid of you," Castiel admits, almost careful in how he parses the words, "Of what you can do. I can't control you— No one can. Lucifer has been riding my "meat suit" for the past—" He doesn't actually know how long it's been. He shakes his head once. The point is— He knows. He knows what that power is capable of in a way he never really appreciated before, and it's given him a certain clarity (or perhaps just bias).

"I'm not afraid to subject myself to you. But I can't risk other people getting hurt just to indulge you."

He wonders what Jack will have to say to that. It would be easy, really, to back Castiel into a corner— Make it impossible for him to say no.