I love to sing to myself. It keeps me calm. It also helps me get through tough situations. I don't enjoy singing in front of others. I don't have a bad voice or anything, I just don't want others to hear me singing.
As I've gotten older I've realized that I'm not an extrovert nor (obviously) an introvert, but I'm an ambivert. sometimes I need space to think, and actually have come to enjoy and even value my alone time.