How did I get a cavity in my tooth?

How did I get a Cavity?
The outer part of your tooth is made of enamel.  From a very young age there are bacteria in your mouth. The most common on your teeth is Streptococcus Mutans.  These bacteria love to eat sugars and make your teeth their home.
The bacteria S. Mutans  digests sugars and carbohydrates and then secrete Lactic Acid.  Acid dissolves enamel.  This slowly forms a hole by eating away at your enamel. Note that the two places people get cavities are in the grooves of teeth and where two teeth touch each other. That is because this is a great place for bacteria to grow and avoid a toothbrush.
The reason brushing and flossing decrease cavities is that by removing the bacteria there is less acid secreted when you consume.
Fluoride in toothpaste or mouth rinses soak into the enamel. This forms a less acid soluble shell that protects your teeth from acid.
Once a small hole forms in your tooth it becomes a great place for the bacteria to hide from brushing and flossing. After a hole reaches a certain size it will only get bigger and can't remineralize. At this point you can have the hole filled by a dentist  so that there is no bacteria home and the hole can stop growing.