The very first main reason vegetarians claim is that they kind of have problem with eating meat/ ethical problem. A lot of people disagree how chickens has been beaten, lived in very tiny cages, or the way these chicken has been slaughtered if they can't produce that much eggs compare to other chickens. Most vegetarians disagree with animals that are not in good condition or environment, like crowded cages. Or they use hormone laden feed so that these animals will grow bigger, faster and therefore produce more, Vegetarians who have these reasons has their own boundaries of course it depends on their beliefs, for an instance, some vegans doesn't take food that has yeast on it or they don't eat specific food.
there are other vegetarians that are called pseudo-vegetarians. They actually eat chicken or fish on a regular basis
Another main reason why vegetarians doesn't like to eat meat, is because meat conflicts with their diet preferences. It simply conflicts on how they eat meat. Like for example, some vegetarians just doesn't like the taste of meat, the texture of it, or maybe because they thought that meat is very high in cholesterol, it has high-concentrations of preservatives, So they don't eat meat because they don't like to.
There are still some minor reasons why they chose to be a vegetarian, I have nothing against it, It's good for your health, and you don't hurt anyone. So i guess being a vegetarian is all about choice.