Vegetarian diets are not new, but today’s world offers many compelling reasons to reduce or remove meat products from our diet.

Human health benefits

People most often cite personal health as a reason for becoming vegetarian, which can be broken down into several parts:

Reducing salt and saturated fat – from a macro/micro-nutrient perspective, reducing sodium intake [which is often high in processed meats] and promoting a favorable lipid balance [shifting toward mono-unsaturated, vegetable-based fats] will reduce the risk of high blood pressure and heart attack/stroke, respectively.

Reducing biological magnification – eating food sources lower on the food-chain will reduce an organism’s exposure persistent toxins in the environment such as heavy metals or pesticides that are damaging or cancer-causing to the human body.

Reducing caloric density – as meat is primarily composed of fat and protein, and does not contain a high volume of water nor fiber, eating a diet high in meat content will likely contain a greater number of calories than a plant-based diet, increasing the likelihood of overeating.

Improving energy conservation and human/animal health

Although the ethical issues in eating animals are separate from the intent of this post, the impact of industrialized livestock farming and their effects on animal health [and consequently human health] should not be ignored.

Energy consumption and efficiency – the main drivers of capital food cost are land area, fresh water, and fuel usage, all of which increase which each increase in the percentage of meat in the average person’s diet. Overall energy demands can be decreased significantly be reducing or eliminating meat, and further reduced by adopting a vegan diet which also excludes egg and diary products. Research suggests an economic inability to sustain a meat-biased diet with an ever-increasing world population.
Additional reading: Sustainability of meat-based and plant-based diets and the environment

Factory conditions and pharmaceutical use – to reduce cost and optimize the supply flow, many low-cost producers use tightly confined environments specialized to cultivate an animal for human consumption at the lowest cost. This often leads to unsanitary living spaces and animals with decreased immune system response. To counteract this threat to the livestock, many animals are treated with antibiotics or other pharmaceuticals which has the externality of potentially increasing immuno resistance and creating more virulent  or drug-resistant strains of bacteria that can infect humans.
Additional reading: CBS News – Animal Antibiotic Overuse Hurting Humans? – Confirmed: 80 Percent of all antibacterial drugs used on animals, endangering human health

Reducing government feed subsidies, providing long-term sustainability

Governments established farm subsidies as a particular remedy – to support farmers in times of overproduction, or when prices fall to a level below cost – an important safety net. Subsidies encourage consumption of more costly goods due to lowering the end-consumer price. A prime example is corn subsidy, which is now the main source of food for livestock [which provides a series of other problems, as many of these animals did not evolve eating corn]. The availability of reduced-cost feed lowers the price at the market, but does not reduce the economic cost of meat on society.

Without debating the benefits or detractors of farm subsidies themselves, it is clear that their distribution is out of sync with the guidelines for a health diet – small portions of meat/eggs/dairy, leafy greens and whole grains.

Due to the high cost and environmental effects of heavy livestock production [in increased greenhouse gases, polluted air and water], the current model does not scale well past the Earth’s nearly seven billion, not including the growing percentage of meat in the average diet.
Additional reading: Eat less meat, save the planet? Livestock nears sustainability limit – Farm subsidies not in sync with food pyramid