Decay.  Teeth can become decayed to the point that they are no longer able to hold a restoration (filling).

Periodontal Disease.  Improper or lack of brushing and flossing leads to plaque and calculus build up along with an inflammation of the tissues surrounding teeth.  This inflammation leads to bone loss to the point where you develop deep pocketing around your teeth, your teeth become mobile and lack sufficient bone for their support.  This can lead to removal of the affected teeth.

Abscess.  Decay and periodontal disease can progress to the point that the tooth and/or surrounding tissues become abscessed (infected).  An abscess can range from a small swelling inside your mouth to severe life-threatening swelling of your face and/or neck requiring hospitalization.

Impacted teeth.  An impacted tooth is a tooth that becomes "stuck" under your gum tissue or bone due to insufficient room for eruption.  Any tooth in your mouth can become impacted, but most commonly wisdom teeth become impacted.  Not all impacted teeth require removal; there are situations when an impacted tooth can be uncovered an brought into position by your orthodontist.

Crowding.  Your orthodontist may refer you for removal of teeth prior to or in conjunction with braces.  This is to allow adequate space for other teeth to erupt or to be brought into position.

Pathology.  Teeth can become associated with cysts and tumors of the jaws.  Most of the time these are benign lesions that tend to erode or destroy bone and require surgical removal in order to stop the process.  On rare occasions these lesions can be malignant.

Trauma.  You may have been involved in some sort of accident ranging from a fall or car accident to an altercation.  Your tooth or teeth may have been injured such that it cannot be repaired and would require removal.