It isn't really a myth or a theory as much as it is fact that a stressed mind will produce vivid dreams, as well as it is more likely that high stress will result in nightmares when dreaming. Losing teeth is a relatively common anxiety that most people have stored at least somewhere, and considering that in times of high stress, we're not likely to take the greatest of care when keeping our body clean and healthy, that makes the idea of messing up your teeth and losing them much more probable to a troubled mind. Now, having actually lost teeth in a brutal confrontation with a floor (it's okay, they're back in, just a bit broken [only three of them]), as well as having several teeth removed 9 years ago (adult teeth were pushing in fast, the baby teeth had to go), I can safely say that the idea of permanently losing teeth is a scary one. It isn't all that surprising that it would be a more common stress induced dream.
Considering you're only 13, it would be a good idea for you to refrain from sexual humor. We don't need Chris Hansen asking any of us to have a seat.