I don't have this very often but I do remember it once in awhile. I figured that maybe sometimes when I sleep on my face I clench my jaw while dreaming sometimes and then my mind plays on that a bit resulting in a tooth getting loose and falling out.

What do teeth falling out in dreams mean in the dreaming community?

I never stress about it while dreaming. Its not really painful, I'm just kind of whatevers about it when it happens.