You are not getting the proper minerals in your diet, or are not utilizing them properly. If you want healthy teeth please research a fellow named Weston A Price, he really knows what he is talking about (he is dead now but his teachings are all over the internet if you search).

Many traditional people across the world never brushed their teeth and didn't have cavities. Its not how clean your teeth are, its how well they are built from the mineral stores in your body. If you are deficient in minerals in any way then your teeth will suffer.

Also, things like fluoride and mercury can make your teeth more susceptible to cavities.