I keep having ulcers in mouth. is there anyway to prevent them? and is there a reason why they keep coming back?

Ulcers: I am presuming that what you are describing are viral in origin. Sometimes they are referred to as aphthous ulcers. They come out anytime. For reassurance, have it looked at by your physician.

Aphthae?: White ulcers in the mouth that cause discomfort especially on drinking acidic things like orange juice are called aphthae; they contrast with viral lesions such as herpes on the lip or herpangina, a one-time disease with many throat ulcers. If you have them often or they are numerous, your physician knows whether to rule out behcet's, lupus, and/or neutropenia.