Why do canker sores form in the mouth?

Unknown: We really don't know why canker sores develop. They do seem to be related to stress in combination with some irritation to the mucous membranes of the mouth. This can be from an accidental scratch with toothbrush, hard food, sharp tooth, etc.

Viral infection: Most people have been exposed to the virus that causes canker sores. It "hides" out in the nerves near the affected area, and stress, trauma or many other factors may cause a recurrent "breakout" of these sores. It should not be a major concern, unless a patient has immune system problems.