Why do people need a root canal?

To treat ...: If the nerve tissue (nerve/blood vessels) within the tooth die, then the toxic products from their death drain through the tip of the root and cause an infection in the bone - leading to bone loss. The root canal removes the dead tissues and seals the canal within the tooth/root to protect the bone.

Dead nerve: A root canal is needed when the nerve in a tooth dies and the patient and dentist desire to save the tooth.

It varies...: Most commonly there is deep decay which enters the nerve space. Another reason may be that too much tooth structure was missing. And another reason could be that the tooth abscessed or died as a result of trauma over the years from decay, preparation for fillings, preparation for crowns, crack propagation, or a blow of some kind.