There are so many different reasons why people go vegetarian or vegan:

Religious or Spiritual Beliefs: There are several religions that prescribe vegetarianism. For example, both Hinduism and Buddhism believe that life is sacred and that humans should not take animals’ lives for unnecessary gratification. Hinduism and Jainism believe that our food shapes our personality; meat is believed to make us aggressive. There is also some speculation that Jesus was either a vegetarian, or that he advocated vegetarianism. This has not been proven true.
Environmental Purposes: Some believe that omnivorousness (noun for “being a meat-eater and a plant-eater”) is not environmentally sustainable. Many times, environmental vegetarians are also economic vegetarians; they believe that the consumption of meat is not economically savvy and that world hunger could be greatly reduced by vegetarianism.
Physiological Reasons: There is debate over whether humans are better suited to be omnivores (consumers of meat and plants) or herbivores (plant-eaters). Great thinkers such as Albert Einstein believed that herbivorousness was simply a part of human evolution that we were beginning to reach. Several studies have indicated that vegetarians are healthier than omnivores (heart health, cancer rates, etc.). Still others believe that humans were never designed to eat meat in the first place; predators LOVE the kill, the idea of raw and bleeding meat, and have the teeth to go along with this love. Most humans do not relish the thought of hunting, killing, and devouring their raw prey.
Psychological Health: Some people find the idea of meat wholly unappetizing. As stated above, humans are not attracted to dead flesh as many carnivores are, and in fact are disgusted by such a site. Douglas Dunn’s famous metaphor states that if a child were given a chicken and an apple, he or she would instinctively play with the chicken and eat the apple. A lion would do the opposite.
Nutrition: Most nutritionists believe that eating lots of fruits and vegetables is healthy for us. Consuming a diet rich in these foods helps reduce the risk of heart disease, stroke, cancer, and more. Also, when we don’t consume human-raised meats, we also do not consume the chemicals and pesticides that the animals consumed during their life. There are many reported health benefits to a vegetarian or vegan diet, and little or no ill effects (especially if one takes vitamin supplements). Nutritional reasons include food safety as well. Over the years, here have been many health scares related to the consumption of meat: Mad Cow Disease (beef), Avian Flu (poultry), Foot-and-Mouth disease (mutton), high mercury levels (farmed salmon), salmonella (eggs), and more.
Social Reasons: People who are raised in a vegetarian household, people married to vegetarians, and people who have close friends who are vegetarians are more likely to become vegetarians.
Ethical Purposes: Many people become vegetarians because they believe that the meat industry is cruel to animals. They may believe that animals have souls, or that animals have inherent rights (like humans do), or they many be opposed to the suffering of other living things. PETA, or the People for the Ethical Treatment of Animals promotes vegetarianism and veganism for ethical purposes.

Wow! Did you catch all that? It’s a lot of information, I know. But I think it’s so very interesting. And again, these classifications don’t exist to make things harder, but simply are a way to describe your personal beliefs. For example, I became a vegetarian for physiological, nutritional, and ethical reasons. That’s to say that I question whether humans were ever intended to eat meat (we can get every amino acid, protein, vitamin, etc. from plants), that I believe my body will be healthier if I don’t eat meat, and that I am opposed to the mistreatment of animals by the meat industry.

So now that I’ve piqued your interest or curiosity, you may be asking “Ok, this sounds great, but how practical is it? If I didn’t eat meat, what the hell would I eat?” Plenty! And I will tell all tomorrow.