Castiel: When Jack was dying, I... I made a deal to save him.

Dean Winchester: You what?

Castiel: The... The price was my life. When I experienced a moment of true happiness, the Empty would be summoned and it would take me forever.

Dean Winchester: Why are you telling me this now?

Castiel: I always wondered, ever since I took that burden, that curse, I wondered what it could be, what... What my true happiness could even look like. I never found an answer. Because the one thing I want... it's something I know I can't have. But I think I know... I think I know now. Happiness isn't in the having. It's in just being. It's in just saying it.