Tooth pain is caused by specific types of bacteria. They produce acid that destroys the tooth's enamel and the layer under it, the dentin.
Many different types of bacteria normally live in the human mouth. They build up on the teeth in a sticky film called plaque. This plaque also contains saliva, bits of food and other natural substances. It forms most easily in certain places. These include:

• Cracks, pits or grooves in the back teeth
• Between teeth
• Around dental fillings or bridgework
• Near the gum line

Acid also can seep through pores in the enamel. This is how decay begins in the softer dentin layer, the main body of the tooth. As the dentin and enamel break down, a cavity is created.
If the decay is not removed, bacteria will continue to grow and produce acid that eventually lead to cavities.

Tooth pain  caught in the very early stages can be reversed. White spots may indicate early caries that has not yet eroded through the enamel. Early tooth pain may be reversed if acid damage is stopped and the tooth is given a chance to repair itself naturally.

Tooth pain caused by cavities that has destroyed enamel cannot be reversed. Most cavities will continue to get worse and go deeper. With time, the tooth may decay down to the root. How long this takes will vary from person to person. Cavities can erode to a painful level within months or years.