There are lots of different reasons why a tooth might need dental work. Cavities are the most common problem. Cavities happen when bacteria eats away a portion of the tooth. The tooth then becomes soft, and infected. The area must be cleaned and filled right away to prevent further damage.
We also see a lot of fractures or cracks. These can happen when the tooth suffers from some sort of trauma (such as being smacked by a baseball), but it can also happen when a patient chews on ice, or something hard.
Finally, we also see cases of extreme wear, especially in patients who grind their teeth, or who suffer from acid reflux disease and thus from tooth erosion.