well its just personal preference i suppose, some people are vegetarians because they don't like the taste of meat or maybe they don't think killing animals is right or it could be religious reasons. but all vegetarians are veggies for different reasons. :)
There are many reasons why vegetarians are vegetarians, one can be due to religious beliefs, which cab be more common in countries like India and Nepal, due to certain Hindu sects, there are Adventists within Christendom who follow a vegetarian diet. Then from there it's a matter of personal choice largely. That's then followed by some who believe humans are vegetarian and meat is something not eaten by vegetarians whiled eggs and dairy are within the acceptable foods. Then there are the rare cases usually due a certain type of tick, that makes the eating of meat not possible *** it causes the equivalent of an allergenic reaction Those who don't eat meat by choice do so in some cases as they believe that meat isn't needed, and that it's wrong to kill an animal as a source of food. Those are the base reasons in a nutshell as to, why vegetarians are vegetarians.
For me, it's because I don't believe animals are often treated in a fair way when they're killed and I've never been a fan of meat. Many others are similar, or religion is a popular reason :)