Joel Berg, president of the American Academy of Pediatric Dentistry Foundation and VP of Clinical Affairs for Philips Oral Healthcare, offers this explanation:

Dental caries is the most prevalent infectious disease in humans, affecting 97 percent of the population in their lifetime. The result of the disease process known as dental caries, getting cavities is a complex and multifactorial scenario. Caries is biofilm-induced, acid demineralization of the teeth, and it requires the right combination of conditions in order to progress. When tooth enamel is subjected to a pH lower than 5.5, it begins to demineralize. Above this so-called critical pH, a mitigating repair process—remineralization—can occur. Remineralization is in turn influenced by the presence of salivary minerals, available fluoride ion and salivary flow rate. It is when the "tug of war" balance leans toward the demineralization side over a period of time without commensurate and compensatory remineralization that the caries process can progress to the point of cavitation and create a visible cavity. This cavity must then be restored using operative dentistry procedures, better known as fillings.

All bacterial biofilms are not alike, however. Although Mutans streptococci and other species have been implicated as primary culprits in causing caries, some people who are infected with these species of bugs dont get cavities. Genotypically different strains of the same bugs exhibit varying levels of cariogenicity. Therefore, its not just the quantity of plaque biofilm present, but the specific strain with which one is infected that is important in predicting who might get cavities.

Diet is another contributing factor to cavity susceptibility. It seems clear that caries-causing organisms prefer sugars—specifically sucrose—as a primary energy source. The metabolism of this sugar into lactic acid causes cavities. Going back to the tug of war scenario between demineralization and remineralization, one can quickly surmise that it is the frequency of sugar exposure in ones diet—not the quantity—that predisposes caries to progressing in an effectively unmitigated manner. Therefore, controlling the number of sugar exposures (consolidating sugar-containing-sweet-eating episodes to mealtimes, for instance) aids the remineralization side of the equation.

A third important factor is salivary flow and composition. Simply said, the more saliva present in the mouth, the better "natural debridement" occurs, cleaning teeth surfaces of caries-causing organisms and the acids they produce. If the saliva is too viscous, however, it may not exhibit the right flow properties to effectively clean affected areas.

Oral hygiene is also an important contributing factor. Our dental professionals continually admonish us to "brush and floss." These oral care regimens must be performed religiously at least daily, and preferably twice a day, in order to be entirely effective. These activities reduce the levels of "local factors" (biofilms and their nutrient warehouses), which helps keep caries under control.

Another factor contributing to the difference in caries manifestation and progression in different people is tooth morphology. Deep grooves on the surface of teeth (molars in particular) make them susceptible to caries, because they entrap biofilms easily and make it harder to remove them.

These are just some of the many factors that contribute to caries progression and ultimately cavities. The diet, hygiene and fluoride regimens are controllable. The strain of bacterial infection, salivary flow and morphology of teeth are less controllable. Regardless, one can see why some people get cavities and others dont.