People don't realize how much of a guessing game cavities really are (when they aren't causing you pain).

I have the opposite problem where my teeth hold up very well. Eventually my dentist is like "we should probably fill those". My first cavities and I am middle aged. Sure, sounds good. After he started drilling he remarked "wow, they are larger than I expected". Afterward I realized my freshly filled teeth felt much better. It had happened so gradually I didn't notice they didn't feel normal anymore.