What causes mouth sores to reoccur?

Depends: That all really depends on the type of sore. Sores can occur for a variety of reasons, trauma, viral infection, autoimmune response, bacterial infection, chronic irritation, and so on. My best guess is that you are referring to canker sores. These are usually caused by reaction to some type of food or stress. They are autoimmune reactions and sometimes are severe enough to require meds.