Although permanent teeth were meant to last a lifetime, there are a number of reasons why tooth extraction may be needed. A very common reason involves a tooth that is too badly damaged, from trauma or decay, to be repaired. Other reasons include:
A crowded mouth
Sometimes dentists pull teeth to prepare the mouth for orthodontia. The goal of orthodontia is to properly align the teeth, which may not be possible if your teeth are too big for your mouth. Likewise, if a tooth cannot break through the gum (erupt) because there is not room in the mouth for it, your dentist may recommend pulling it.
Infection
If tooth decay or damage extends to the pulp -- the center of the tooth containing nerves and blood vessels -- bacteria in the mouth can enter the pulp, leading to infection. Often this can be corrected with root canal therapy (RCT), but if the infection is so severe that antibiotics or RCT do not cure it, extraction may be needed to prevent the spread of infection.
Risk of infection
If your immune system is compromised (for example, if you are receiving chemotherapy or are having an organ transplant) even the risk of infection in a particular tooth may be reason enough to pull the tooth.
Periodontal (Gum) Disease
If periodontal disease -- an infection of the tissues and bones that surround and support the teeth -- have caused loosening of the teeth, it may be necessary to the pull the tooth or teeth.