Answer:
Ethical, health, enviromental, political; to help feed the world and reduce cruelty. Many are vegetarian for spiritual reasons as well.
Another Opinion: I personally don't feel that there are many reasons to avoid eating meat, poultry, and fish in moderation. Many people throughout the world enjoy doing so, and these are a good source of protein. No one person knows for certain what animals and fish feel - so long as they are handled humanely that is what matters. Not everyone agrees that vegetarianism is a healthier or more ethical diet, and so no one should feel obligated to follow this way of eating. It is not for everyone.
More "pro" reasons, in more detail--
Animals and fish want to survive, as do all creatures; it shows in their actions. Most slaughterhouse conditions and feed lots where animals are raised in mass conditions are not humane. Being forced or "obligated" to be vegetarian is not a reason to be one; it is a conscious choice, and not an easy one if you're used to eating meat. Compassion for animals is the basic reason to be a vegetarian. Vegetarians also live longer as they tend to avoid many of the food based diseases which are the number one killers of people in the US (and probably other "modern" countries).
Concern for the environment is another reason to become vegetarian; meat eating takes vast resources of feed and water that strain the environment. Livestock produce more greenhouse gases than all the cars in the world combined! Pollution from hog farms and cattle feedlots is an environmental problem.