A tooth can never grow back or heal itself on its own once it has been broken or decayed. Bacteria eating away at the outer layer of your teeth, known as the enamel, causes tooth decay. Although sugar specifically causes decay to occur faster, everything you drink and eat contributes to tooth decay overtime. Saliva is the best defence against the buildup of bacteria, but it can only do so much.
Since the protective enamel is no longer present in broken or cracked teeth, they are more susceptible to decay. If a broken tooth is not repaired or if decay is not properly removed the cavity will continue to grow and could reach the nerve of the tooth. This would then cause the need for a root canal or extraction of the tooth completely.