Doctors aren't absolutely sure what causes canker sores, but there are multiple factors that contribute to canker sources even within individual people. The leading theory is that canker sores are caused by some sort of mouth irritation. Canker sore prevention is key to avoiding these painful mouth sores.  Even though there is no sure-fire 100% effective method for canker sore prevention, there are herbal remedies that can alleviate the pain and discomfort caused by canker sores.