There are many reasons that people get sores on the insides of their mouths, but there is more information that needs to be understood before we can completely answer your question. One of the most common kinds of sores that people get in their mouths are called apthous ulcers, or canker sores. While we don't completely understand what causes them, they tend to appear more frequently when people are under stress or their immune systems are not functioning optimally. They are very tender, but usually go away quickly by themselves, and can appear anywhere in your mouth. They can be treated with topical anesthetics which are available over the counter. If they are really bothering you, they can be cauterized with a chemical in your doctor
's office, and some people are prescribed different medications which can help. While cautery doesn't decrease the healing time, it does decrease the pain. While canker sores are the most common type of sores in the mouth, there are other kinds as well, some of which can be serious. Please make an appointment with your doctor to be examined and discuss this issue in detail. Good luck!