Why are Jains vegetarian?

Jains believe in ahimsa, or nonviolence. To stop the cycle of reincarnation, Jains believe that they must follow the path of ahimsa. If they do not behave as their religion dictates, they believe that their next life could be as a lower animal. Thus they do not kill any animal because it could have been their ancestor, etc. But primarily, because they believe in preventing the suffering of all who feel pain.