The document discusses three main causes of tooth mobility: 1. Gum disease, which results in loss of alveolar bone and periodontal ligament support for the tooth. Splinting or treating the underlying gum disease can sometimes reduce mobility. 2. Trauma from occlusion, such as from excessive biting forces, fillings, or grinding. This causes inflammation in the periodontium and loosens the tooth. Removing the source of trauma may help. 3. Periapical infections from dead or infected teeth, which cause inflammation and loosen the tooth. Treating the infection, such as with root canal therapy, can reduce mobility.