Root canal treatment is usually required when the pulp tissue within the root canal of the tooth has become inflamed and/or infected with bacteria. This is commonly a result of decay, repeated dental procedures and/or hairline cracks or chips on the tooth (which may not be readily detectable to the human eye).
The tooth in question may present with symptoms such as sensitivity to hot/cold, swelling and/or tenderness of the overlying gum. However, it is not unusual for there not to be any pain from the tooth in question.