Brushing does not clean: there are many dental plaque bacteria play around neck. More bacteria when carbohydrate foods.

Bacteria ferment carbohydrates in the diet of beings acid. Acid soluble mineral salts of the enamel. Long day caused deep into the tooth.

Defects of enamel, there are two main reasons: 
Soluble in acid enamel by custom made ​​component minerals in the enamel surface 
The enamel surface more sunken, to close the food but hard brush to clean. Nutritional food source for bacteria, acid causes cavities in the central groove. 
Therefore brushing alone is not enough to keep teeth decay