There are many reasons why people get cavities. The most obvious is sugar which feeds plaque which grows and then demineralizes the enamel. In simpler terms, the acid in the plaque eats away at the protective covering on your tooth.
What you might not know is that there are other ways the plaque can get out of hand and remove precious enamel. Also, there are ways to preserve the hardness of your enamel and new technologies to help replenish it.