Why do people get reacurring canker sores in there mouth?

Inflammation. Recurrent canker sores are inflammation and ulcers of the lining of the mouth .These may be triggered by stress, food sensitivity. A good anti-inflammatory treatment is dgl - deglyzzrihinated licorice which calms the oral mucosa when chewed. Also reflux from the stomach can be associated with canker sores .
Canker sores. Canker sores are most likely caused by a virus that lays dormant within your body. During times of stress the virus can cause one of these sores. Also, toothpastes and mouthwashes that contail sls, or sodium lauryl sulfate, can increase the incidence of sores in those who have had them before. Finding oral hygiene products without sls will help decrease the incidence of canker sores.
Viral. Although these are hindrances; they are just viruses at the end of the day. Stressors usually exacerbate them; and they can be nipped in the bud if caught early enough. Once your body decides to turn it's immune system "on them" then you will never get another one.