Dancing.

How did moving ones arms and legs in various directions to a beat become convention?
What possesses people to move the way they do to music?
It's like, oh, hey, I like this song, maybe if I start moving around, it'll show my appreciation more.
And why do certain people have to be born with the pitifully embarrassing defect of being bad at said movement?
Luckily, I'm not one of them, but I always feel bad when people are, because it's so pointless and yet so important.
Also, who came up with which movements are conventional dancing?! Like why does shaking your shoulders uncontrollably look normal, but shaking your legs doesn't? Huh?