In retrospect, Cas knows how horribly he'd always explained himself. It was almost a thing lost in translation; moving from Grace and Enochian to a human mouth, through English language, it was as if everything he needed to get across fell to the wayside and left him speaking the bare bones. 'I did it for you' contains so much more meaning in Enochian, in Heaven, than it did here on Earth.

Words are so insufficient sometimes, but they're all Cas has now. "It's in the past. My reasons, your reasons. We should leave them there." It's too much to explain and it feels as if it would hurt coming out. Cas doesn't want to lay himself raw and open for anyone, not even Dean.