I don't know why this happens, I used to think it's anxiety. It's worth noticing that it seems to run on families too, but perhaps this is related to mirroing rather than genetics. All I know is this is difficult not just for those whose body is shaking but those around them too. I swear with my girlfriend is like we live in a perpetual earthquake and sometimes I even have to tell her to stay still because for extended periods she can actually make me dizzy. Like she will be doing this for a long time while we are on bed right after I had lunch or something like that.