What is the main cause of cavities? Cavities are permanently damaged areas in the hard surface of your teeth that develop into tiny openings or holes. Cavities, also called tooth decay or caries, are caused by a combination of factors, including bacteria in your mouth, frequent snacking, sipping sugary drinks and not cleaning your teeth well. Why are my teeth rotting even though I brush? Tooth decay also occurs when foods containing carbohydrates become trapped between teeth and are not completely removed with brushing and flossing. Major causes of tooth decay are sugary, sticky foods and beverages. The more sugar consumed, the more acid, which gets produced leading to decay.