Root canal treatment, is needed when tooth pulp as shown in attached figure becomes inflamed or infected. The causes could be deep tooth decay, repeated dental procedures on one tooth (replacing a large filling, for example), or traumatic damage such as a crack, chip or even a root fracture. Gum disease can also give rise to root canal problems necessitating root canal treatment.
Any of these issues can result in acute inflammation of the pulp, which causes swelling and pressure inside the tooth, leading to tooth pain and, eventually, irreversible damage to the pulp. Once the pulp dies, the pain may subside initially, sometimes to return as an painful infection spreading into the periapical tissues, particularly the bone. It could also become a chronic (long-standing) infection with symptoms ranging from mild to severe.