This is typically due to resorption. Within the first year of tooth loss you can lose up to 25 percent of your bone density in that particular area. It only continues from there. That is because teeth and roots provide stimulation to the jawbone and that stimulation is a signal to the brain that your jaw is serving its primary purpose. When teeth fall out, that stimulation is reduced or eliminated completely. As a result, the body begins to leach calcium from the jaw for use elsewhere. If you know that you are going to lose a tooth, do not hesitate to schedule an appointment since there are certain procedures that can prevent or limit the amount of resorption.
Other reasons for a loss of bone density is periodontal (gum) disease, developmental defect, injury or decay and infections. Visiting the dentist on a regular basis for preventative care can help to reduce these risks.