Why am I always shaking?

It's not serious shaking, and it's always there. Mainly in my hands. It's actually a trait in my family, and my grandfather and some of his siblings have it too. I can't hold my hands completely still, but it has nothing to do with PD or anything like that. We just shake in varying degrees. But we can still drive, draw as straight a line as the next person, so it doesn't interfere with our lives. So my question is, why do we shake? I mean, it looks like I'm always scared, shaking as if I was just scared. And I'm looking for a reason why. So far, my research has come up with no results that I can use. And that's why I'm asking here. Any ideas?