I think it can also be to avoid the negative situation where someone feels left out or unlikeable. Or in the case of children, to let them know you're having fun like they are. A split second of recognition (or the lack of it) can make a big difference in someone's mood.

I grew up around a lot of stone-faced adults, and was always wondering if I made them mad or said/did something stupid. I don't want to make others feel that way.