He is the one who has made it possible for us to get right with God. What Jesus did for us, the angels understand, is a big deal, and worthy of a big celebration. So the angels have a big reason to celebrate, and they celebrate it in a big way. I am much better at whining than I am at parties. Parties are too messy.
Parties are work.
But some of you know how to put on a great party. You know how to celebrate. You really know how to have fun, and how to help the rest of us whiners have fun. When it's someone's birthday, you put on a party for them. When a child graduates, you celebrate.