When cavities first form, they are usually quite small and you won’t feel anything. If you do feel pain from tooth decay, that's a real cause for concern. It could indicate that there is already infection in the pulp of your tooth. If that’s the case, a filling won’t solve the problem; you might need a root canal and a crown.
Most often, cavities develop in the back molars. Those teeth are more prone to cavities because the groves in them provide a good place for plaque to take up residence. As plaque sits there, it mixes with acids from the foods you eat and causes tooth decay. The outer layer of tooth enamel is eaten away gradually. If the decay is left unchecked and reaches the inner layer of the tooth, the decay progresses much more quickly. This level of decay can lead to sensitivity or pain.
This is just another reason why it is so important to have regular dental examinations at least twice each year. If a visual check from your dentist doesn’t spot early signs of tooth decay, a digital x-ray will likely show a small shadow. The cavities can be filled before you feel anything.