Cavities usually start in the outer layer of the teeth called the enamel. This outer layer is no longer alive. This is similar to your hair and fingernails that do not naturally heal on their own. The enamel can be treated with fluoride to help strengthen it. Due to the lack of actively dividing cells, lost tooth structure in the enamel can not naturally replace itself.
Some tiny cavities can be treated with good oral hygiene and the use of fluoride treatments. If the decay has gotten to the deeper layers of the teeth, such as the dentin, dental treatment like a filling or a crown are needed.
By allowing the decay to get larger and larger, you only give the bacteria more opportunities to grow and cause more problems.
The longer you wait, the worse it will typically get. Good oral hygiene can keep it from progressing quickly for tiny cavities, but you will not be able to reverse a cavity that has broken through the enamel.
The longer you wait, the deeper and broader holes will form in the tooth as you go through each layer of the tooth, enamel, dentin, and pulp (the nerve and blood supply). The bacteria will be able to travel faster and faster. This will lead to more damage to the tooth and more pain.