Why did they sing? You’ve

Why did they sing?
You’ve seen them do it on poorly drawn greeting cards and towering works of art, but have you ever thought about why the angels sang at the Nativity? I really hadn’t before a few weeks ago. It’s strange, though, that the angels would sing — why would they be so happy that God had entered the (to them, lower) material world? I have my own ideas, but I’m curious to see what you all think.