Well have you ever had a dream in which you tried to move your arms or legs (in your dream) but couldn't? Or thy moved really slowly? The reason you can't sometimes is because when you're deeply asleep you're pretty much paralised, and this translates to your body in your dream.

People often wake up with a horrible taste in their mouth in the morning and immediately want to brush their teeth (like most people). This is because we don't swallow when we're deeply asleep. So I think when we're asleep we can still feel that strange feeling in our mouths and it translates over to our teeth in the dream and our subconscious confuses the dream with our psychical sensations and mixes the two; hence dreams about teeth.

Hmmm... I can't explain it very well.. lol