I sing too. I can't not sing if there is music playing, but I tend to either sing inaudibly or in my head when other people are around.
So, if you were a fly on my wall you would see me wandering around singing to all the TV commercials, kids programs, TV show theme songs, as well as music from my mac. Musical movies are the best! It's really fun to sing tunes from Mary Poppins for instance. When I'm really happy tunes like 'Let's go fly a kite...' pop into my head and I have to stifle myself from bursting out into song (unless I'm alone).
I also have a little morning ritual where I sing opera while I froth milk for coffee (it's boring waiting for it to froth). At the moment I've been singing Pie Jesu - from Gabriel Faure's Requiem.
p.s. I dare you to start singing opera while you go to the toilet! The little room acoustics/sound waves bouncing off tiles make it sound cool!
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Hope is the thing with feathers, that perches in the soul, and sings the tune without words, and never stops at all. -emily dickinson