What causes canker sores in your mouth?

They're caused by all sorts of things. Including a faulty immune system, minor injury to the mouth, an allergic response to certain bacteria in your food, a diet lacking vitamin B-12, zinc, folic acid or iron, food allergies or sensitivities, certain foods - chocolate, coffee, strawberries, eggs, nuts, cheese, pineapple, hormonal change during menstruation, toothpastes and mouth rinses containing sodium laurel sulfate, and emotional stress. I have one now come to think of it, and it's killing me.