Perhaps it was annoying, being pestered by your parents to brush as a child, but very often the most obnoxious advice is the best. Few things are as valuable as your teeth, and as you get older, the chance increase of damaging or even completely losing this precious gift.
The most obvious point is that teeth are valuable for chewing. Without a full set of choppers, the food selection available to you becomes fairly limited. Raw vegetables must be boiled and steam, and some of the best foods, such as steak, become impossible. Even if you do not loose all your teeth, tooth decay may contribute to sensitivity, and even a few lost teeth can make chewing problematic. Either you must chew with a single side of your jaws, or you must pick out bits later.
Less obvious but equally important, teeth are important for speech, particularly the front teeth. Miss a few front teeth, and loose the ability to enunciate certain letters. Missing teeth also carries a negative stereotype, suggesting poverty and a crude existence. At the very least, denture replacements can be annoying to put up with, and without your teeth, your face seems older and less full.
The secret of caring for teeth begins with knowing what not to do. Eating too many sugary foods erodes teeth, because residual sugar is quickly decomposed by bacteria, which releases acid as a consequence, and that acid strips minerals from the tooth enamel. Wheat products are just as bad, because simple carbs are as easily digested, and bread tends to stick to the teeth. It is an important reason to brush after every meal. Beyond the dinner table, take care of what you do with your teeth. Avoid hazardous situations that might knock out teeth, and do not drink too many sugary drinks and snacks.
Properly brushing is important, but so is flossing. Many people forget to floss regularly, but it is between the teeth that food particles stick most tightly. Bacteria colonies may develop between the teeth, and eventually wear through the enamel because they are never removed. A large percentage of cavities begins between the teeth, and may have been prevented by proper flossing.
Just as important to tooth health is not to smoke, and to drink infrequently. These and other drugs can damage the gums, and unhealthy gums invites infection and tooth decay. Drinking may spur acid reflux, which erodes both the esophagus and teeth.
And, of course, routinely visit your dentist.