This is response to a journal post by theggreen1's on OkCupid, where he basically asks, "What's the deal with vegetarians? Don't they know we have incisors for a reason? Since virtually all life sustains itself from the death of another creature, the vegetarian 'holier than thou' attitude is completely asinine." I am paraphrasing, but that is the general thrust of it.
This was my response.
For many vegetarians it is not necessarily so much a matter of trying to reduce the net amount of death in the world as it is trying to reduce the amount of animal suffering that results from their eating habits, or because they want to dampen their negative impact on the environment.
The majority of animals raised for eating in the U.S. live out most of their lives in an industrial CAFO (concentrated animal feeding operations), which is about as miserable a "habitats" that you can imagine for these animals, and don't even remotely reflect the animal's natural environment. It's standard procedure, for example, for animals to have their beaks or tails clipped to prevent them from maiming their neighbor animals when they go stir-crazy.
In some cases (particularly in the case of cows), their diet consists of food that they would never touch in nature. Ruminants like cows normally subsist entirely on grass ... in CAFOs, they are fed copious amounts of corn mixed in with their grass, supplemented by various animal fats and heaping quantities of antibiotics (a great way to breed super-germs), because without them and other medical care, their stomachs would explode (well, maybe not quite that dramatic, but with more or less the same consequences).
The other possible motive for adopting a vegetarian lifestyle that I mentioned has to do with sustainability. Growing a monoculture of corn on an industrial farm/plantation just to feed it to animals is hugely inefficient in terms of calories netted vs. calories spent. It's not even a particularly efficient way to produce protein. Add to that these farms and CAFOs tend to pollute the air, consume and pollute the local water supply and rely heavily on the burning of fossil fuel and it is pretty clear that this is ecologically not a very sound way of doing things and will ultimate spell some problems. Eating "lower on the food chain" as vegetarians do alleviates some of these issues.
I'm not a vegetarian myself, so I'm not trying to convert you or anything. I'm just explaining why vegetarians eat the way they do even though nature obviously has "intended" us to function as omnivores. We are well adapted to eating a variety of food and doing so benefits us nutritionally, under normal, or perhaps "natural" circumstances. The additive-laden fast food diet of your average American is no more "natural" than vegetarianism and is almost certainly no more healthy.
If you're wondering where I'm getting all of this, most it comes from the fact that I've recently read The Omnivore's Dilemma by Michael Pollan. It is a most illuminating book if you're interested in understanding how the food you have come to ingest got to your dinner plate, though most people may feel better off not reading it unless they're willing to go to some lengths to make changes to their diet.