I enjoyed seeing the uncomfortable reactions of people when we had been asked to sing them happy birthday
Why did you enjoy them being uncomfortable?
It was funny because they looked so miserable.
Taking pleasure in seeing other people have an uncomfortable reaction isn't very nice. If you do this then other people will think you're insensitive and mean. You should try to empathize with others when they're unhappy, not take joy in it.
It's just really funny to me when I see their face.
I'm sure you wouldn't want people doing that to you.  When you think people's discomfort is funny it makes you sound very immature.