I've always assumed it is because you brush your teeth right before bed. It is a very common thing for people to dream about. Think about it, you spend 5 minutes right before sleeping in front of the mirror staring at your teeth. Brush, floss, mouth wash, the whole time thinking "I better take care of my teeth so they don't fall out. Don't let those teeth rot and fall out."

That is exactly how I incubate dream content.