"Happiness isn't in the having, it's in just being.." ~Castiel
Misha's interpretation: "I think being in that moment, being present in that moment is ultimately like the only way to be happy. Not projecting yourself into the future and thinking about what you're going to get, what you're going to accomplish, what you're going to do and not dueling in the past either celebrating something that happened in the past. I think that what Cas said is actually pretty on the nose what happiness is. It's just being in the present."
I never interpreted it this way 😳 That means Castiel was focused on that particular moment. That moment, when he stopped denying his love for Dean, when he decided to openly talk about his feelings, when he said those three words out loud. He focused on his last moment with the man he loved and that was his happiness.