There are many reasons why people become vegetarians, but they predominately fall within three categories: health benefits, ethical responses, or in support of environmental sustainability.
Although there are definitive pros and cons to both sides of the argument, it is a very personal decision. With this as my premise, I will try to summarize and simplify the main points before I put in my two cents. Please reference this article for further credibility.
Here are the claims: vegetarianism proponents argue that eating meat "harms health, wastes resources, causes deforestation, and creates pollution (1)." What's more is the ethical stance that killing animals for food is cruel and unnecessary because non-animal food sources are plentiful.
To counter, opponents argue that consuming meat is healthful, humane, and that "producing vegetables causes many of the same environmental problems as producing meat (1)." In addition, they cite that humans have been eating and benefiting as a means of evolution from meat consumption for over 2.3 million years.
These three articles expand and challenge the former claims: Why Become A Vegetarian, Why I Am Not a Vegetarian, and My beef isn't with beef: why I stopped being a vegetarian.
After reading the articles I have come to these conclusions: eating meat and vegetarianism both have draw-backs in regards to health (vitamin and protein deficiency vs. heart disease susceptibility), and ethically one can not presume to say definitively that killing animals is "wrong" or "cruel", when it is part of the natural cycle of life and a component of human evolution. Once these issues are put aside as impasses, we can examine another approach to vegetarianism, one that is slightly more apropos as a current issue: sustainability.
My third article described one woman's switch from radical vegetarianism to a small farm operator raising (and killing) chickens, pigs, sheep, and others. Woginrich became a vegetarian in response to the misuse of resources, pollution, habitat destruction, animal hormone pumping, and environmental liabilities caused by the poor farming practices of the commercialized American meat industry. She says that "to be vegetarian is to be a pacifist, avoiding the fight against animal cruelty. Eat meat from sustainable farms, and we will win."
I have to agree with her stance. Vegetarianism for animal activism is a personal ethical decision. Vegetarianism for "heath benefits" is dubious. However, if you become a vegetarian in response to the negative practical aspects of the industry, I believe your efforts can be redoubled by supporting small farms that promote the humane treatment of animals as well as environmental sustainability.
So that is my opinion. I know this topic is controversial, so you will probably have your own thoughts. If my sister read this, she would be pretty outraged at my "moral apathy" (she is a stoic vegetarian who sends annual donations to PETA). Feel free to unload ideas.