When the pulp is diseased or injured and unable to repair itself you need a rootcanal. The most common cause of a root canal is a fractured tooth or a deep cavity.
Root Canals are basically the last ditch effort. If you've have that tooth worked on several times and it keeps getting cavities then your may need a root canal.
The symptoms I had when I needed a root canal were severe almost unbearable pain. I also had an abscess which was a infection that had a hard painful knot.
