Why do people choose to be vegetarian?

Health - A healthy vegetarian diet can decrease the risk and symptoms of many chronic diseases such as heart disease, some forms of cancer and type 2 diabetes. It is also associated with longevity. A major Chinese study of 6,500 people (The China Study) showed that the less animal products were eaten, the healthier people were.
 
Religion - Some of the major world religions discourage  adherents from eating meat. Hinduism, Buddhism and Taoism all hold  vegetarianism as a way to promote non-violence which is in turn associated with spiritual development. If Kosher or Halal meat isn't available, observant Jews and Muslims will eat a vegetarian diet. Other religious groups which favour a vegetarian diet include Seventh Day Adventists and Jains.
 
Environment - A great deal of research indicates that large-scale meat and poultry production is environmentally unsustainable. The United Nations Food and Agriculture Organization report, ‘Livestock’s Long Shadow' concludes that global animal agriculture contributes more greenhouse gas emissions (in CO2 equivalents) - an astonishing 18% of the total - than all forms of transportation (13.5%). Another study from the University of Chicago found that the average American diet requires the production of an extra ton and a half of CO2-equivalent - in the form of both carbon dioxide, methane and other greenhouse gases - compared to a strictly vegetarian diet.
 
Animals - Ethical vegetarians are concerned about animal cruelty. Over 60 billion animals are raised, transported and slaughtered each year. Many of these live in appalling conditions and are subject to cruel treatment through transport and during slaughter.
 
Food Safety and Security - There have been many large-scale recalls of meat contaminated with E. coli and salmonella while many local incidents of food poisoning through improperly cooked meat and eggs have been reported from restaurants and take-away food outlets. The vast majority (around 70%) of all beans and grains produced by agriculture are fed to animals and not people.

Firstly let me state, I eat meat, probably too much. However, I have a friend who is a vegetarian and his experience has been very positive. He struggled with weight and several on-going health issues. Going meat-free has solved most of these so he is a zealot for the cause. 

As I said, I'm not a vego but am cutting down on processed meats and meat in general. Too much meat has been shown to be corellated with cancer so I'm easing off a bit. Cheers

There are multiple reasons why people choose to become vegetarian (or even vegan). Some people do it for just one reason (for example, some people do it to protect their health), while others do it for varied reasons. Here are four reasons why people make this choice (and why other people might decide to make this lifestyle change now).

Health: Many people choose to go vegetarian or vegan for the health benefits this lifestyle has to offer. While it is possible for a vegetarian to eat unhealthy foods (such as eating foods made with trans fats or foods with sugar and salt), by and large most vegetarians have healthier diets. It isn't possible to get fibre through meat or dairy products, but vegetarians usually eat lots more fibre than their meat-eating counterparts. Furthermore, cholesterol comes from eating animal products, and people who eat less or no animal-based foods will benefit from this diet. Finally, overall, vegetarians and especially vegans usually weigh less than people who eat meat.

Environment: There are several ways in which a vegetarian diet protects the environment. Excessive amounts of water are used in raising beef and other livestock, water that could be used for other purposes. Furthermore, cattle that are raised for consumption produce large amounts of methane, and methane is a greenhouse gas that contributes to global warming. Meat-eating diets also indirectly contribute to deforestation: land is cleared for either the purpose of raising cattle or for growing grain for said cattle. Many people feel good about going  vegetarian when they know what they're doing for the environment.

Ethical Reasons: Still other vegetarians choose to adopt a meatless lifestyle because they care for animals. Many people don't want to eat something that had to die just so they could be nourished; in addition, vegetarians  don't want any animal exploited for any reason, and they often equate eating animal products (or wearing things made from animals, for example) to enslavement.

Global Food Shortage: As was mentioned earlier, raising cattle requires massive amounts of water that could be used elsewhere; more than that, though, the amount of grain grown to feed cattle could be used to feed starving nations. According to The Vegetarian Society, more than one-third of the grain grown worldwide is fed to animals. Imagine how many people those grains could feed.

There are even more reasons why people choose a meatless lifestyle. While many people resist removing meat (or all animal products) from their diet, just these four reasons show why people should strongly consider making these dietary changes - for their health, the environment, their conscience, and the global community.

Great answer Arlene! Yes, there are many reasons why people decide  to avoid eating animal products. A well balanced vegetarian diet certainly does have many advantages - reduced risk of developing lifestyle diseases such as type II diabetes and cardiovascular disease.  Increasing vegetable intake has also been shown to give the skin a healthy glow which is noticable to the naked eye!  Creating tasty plant based meals does require a bit of extra effort in the kitchen, but with all the beautiful vegetables, legumes, fruits, nuts and seeds available to us in Australia, there is no excuse to increase the number of meat free meals eaten each week.

Arlene has answered this question really well and agree that these (health, environmental, religious and ethical) are the main reasons people choose to adopt a vegetarian diet.

The evidence for the health and environmental benefits of a vegetarian diet is now strong, and even if someone isn't ready to make this change, just eating less animal foods and more plant foods can benefit both your health and the environment. 

This is the idea behind movements such as Meat Free Mondays http://foodwise.com.au/meat-free-mondays/homepage/ which encourages people to take a day off eating meat and to enjoy some tasty plant-based meals.