system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do teeth not repair themselves after cavities?
gpt
Cavities form when bacteria (or plaques, which are biofilms or special colonies of bacteria) secrete acids and other waste products that demineralize your teeth, leaving a hole essentially. For as long as the plaque is still there, the bacteria will continue to secrete acids and if your body can't *re*mineralize as fast as the bacteria *de*mineralize, the cavity will grow. Essentially the answer to your question is because there's usually still active tooth decay going on.