genetic predisposition,vit-b12 folic acid iron deficiency(20%of cases),stress,hormonal change or disturbance like puberty and menopause,some git disorders
i usually suffer from these ulcers when i m in stress , all u can do is to avoid spicy and sharp food +
orabase gel put it on the ulcer only b4 eating + anti inflammatory mouthwash and chlorohexdine mouthwash
but remeber u need also to check ur dentist as i cant diagnose u online also he may ask for special investigations to exculde anemia