Aizetsu admires you, Upper Moon One. Nobody has taken your spot in hundreds of years and he's only gotten to interact with you a few times. Hantengu never lets him out and it makes him sad because all he wants to do is express his admiration for you.

You're the only thing that doesn't make him sad, and when you saved his life and the other manifestation's lives he was finally able to kneel to you and express himself to you. Even though you ignored him he didn't feel discouraged. He'll get your attention one day.