What causes mouth ulcers to hurt so much and how can you prevent them?

No one knows...: ...For sure what causes them or why they hurt, though they frequently afflict people who are fatigued or stressed out or who have poor diets. People taking certain drugs for rheumatoid arthritis may be more prone to developing them. And heredity is a factor, too. Prevent them by eating well, get plenty of sleep, and avoid citrus fruits, which seem to be particularly good at causing them.

Penetration: Ulcers hurt so much because they penetrate to areas of the body that are protected by layers to, where nerves are located. They are caused by viruses that come out when we are debilitated. We have the viruses at all times under our skin but they only cause damage when our body defenses are down due to sickness, overexertion , etc. Take care of yourself as best you can, but sometimes they happen.