Have to love the writing from St. Benedict of Nursia in today's appointed readings in Treasury:
"Therefore, let us consider how it benefits us to behave in the sight of God and His angels, and let us then STAND TO SING, that our minds may be in harmony with our voices."
We stand to sing because we know we are worshipping in the presence of God and are surrounded by His holy angels as we gather together in His name.
P.S. Okay, okay, I'll also fess up: I HATE to sing sitting down as a musician; you can't get the air properly into your chest that way, or at least it is harder to do. Standing and singing - well, they just go together.