Besides being an expression of compassion for animals, vegetarianism is followed for ecological and health rationales

Reasons

In the past fifty years, millions of meat-eaters -- Hindus and non-Hindus -- have made the personal decision to stop eating the flesh of other creatures. There are five major motivations for such a decision:

1. The Dharmic Law Reason
Ahinsa, the law of noninjury, is the Hindu's first duty in fulfilling religious obligations to God and God's creation as defined by Vedic scripture.

2. The Karmic Consequences Reason
All of our actions, including our choice of food, have Karmic consequences. By involving oneself in the cycle of inflicting injury, pain and death, even indirectly by eating other creatures, one must in the future experience in equal measure the suffering caused.

3. The Spiritual Reason
Food is the source of the body's chemistry, and what we ingest affects our consciousnes, emotions and experiential patterns. If one wants to live in higher consciousness, in peace and happiness and love for all creatures, then he cannot eat meat, fish, shellfish, fowl or eggs. By ingesting the grosser chemistries of animal foods, one introduces into the body and mind anger, jealousy, anxiety, suspicion and a terrible fear of death, all of which are locked into the the flesh of the butchered creatures. For these reasons, vegetarians live in higher consciousness and meat-eaters abide in lower consciousness.

4. The Health Reason
Medical studies prove that a vegetarian diet is easier to digest, provides a wider ranger of nutrients and imposes fewer burdens and impurities on the body. Vegetarians are less susceptible to all the major diseases that afflict contemporary humanity, and thus live longer, healthier, more productive lives. They have fewer physical complaints, less frequent visits to the doctor, fewer dental problems and smaller medical bills. Their immune system is stronger, their bodies are purer, more refined and skin more beautiful.

5. The Ecological Reason
Planet Earth is suffereing. In large measure, the escalating loss of species, destruction of ancient rainforests to create pasture lands for live stock, loss of topsoils and the consequent increase of water impurities and air pollution have all been traced to the single fact of meat in the human diet. No decision that we can make as individuals or as a race can have such a dramatic effect on the improvement of our planetary ecology as the decision not to eat meat.

History

The book Food for the Spirit, Vegetarianism and the World Religions, observes, "Despite popular knowledge of meat-eating's adverse effects, the nonvegetarian diet became increasingly widespread among the Hindus after the two major invasions by foreign powers, first the Muslims and later the British. With them came the desire to be `civilized,' to eat as did the Saheeb. Those atually trained in Vedic knowledge, however, never adopted a meat-oriented diet, and the pious Hindu still observes vegetarian principles as a matter of religious duty.

"That vegetarianism has always been widespread in India is clear from the earliest Vedic texts. This was observed by the ancient traveler Megasthenes and also by Fa-Hsien, a Chinese Buddhist monk who, in the fifth century, traveled to India in order to obtain authentic copies of the scriptures.

"These scriptures unambiguously support the meatless way of life. In the Mahabharat, for instance, the great warrior Bheeshm explains to Yuddhishtira, eldest of the Paandav princes, that the meat of animals is like the flesh of one's own son. Similarly, the Manusmriti declares that one should `refrain from eating all kinds of meat,' for such eating involves killing and and leads to Karmic bondage (Bandh) [5.49]. Elsewhere in the Vedic literature, the last of the great Vedic kings, Maharaja Parikshit, is quoted as saying that `only the animal-killer cannot relish the message of the Absolute Truth [Shrimad Bhagvatam 10.1.4].'"

Scripture

He who desires to augment his own flesh by eating the flesh of other creatures lives in misery in whatever species he may take his birth. Mahabharat 115.47

Those high-souled persons who desire beauty, faultlessness of limbs, long life, understanding, mental and physical strength and memory should abstain from acts of injury. Mahabharat 18.115.8

The very name of cow is Aghnya ["not to be killed"], indicating that they should never be slaughtered. Who, then could slay them? Surely, one who kills a cow or a bull commits a heinous crime. Mahabharat, Shantiparv 262.47

The purchaser of flesh performs Hinsa (violence) by his wealth; he who eats flesh does so by enjoying its taste; the killer does Hinsa by actually tying and killing the animal. Thus, there are three forms of killing: he who brings flesh or sends for it, he who cuts off the limbs of an animal, and he who purchases, sells or cooks flesh and eats it -- all of these are to be considered meat-eaters. Mahabharat, Anu 115.40

He who sees that the Lord of all is ever the same in all that is -- immortal in the field of mortality -- he sees the truth. And when a man sees that the God in himself is the same God in all that is, he hurts not himself by hurting others. Then he goes, indeed, to the highest path. Bhagvad Geeta 13.27-28

Ahinsa is the highest Dharm. Ahinsa is the best Tapas. Ahinsa is the greatest gift. Ahinsa is the highest self-control. Ahinsa is the highest sacrifice. Ahinsa is the highest power. Ahinsa is the highest friend. Ahinsa is the highest truth. Ahinsa is the highest teaching. Mahabharat 18.116.37-41

What is the good way? It is the path that reflects on how it may avoid killing any creature. Tirukural 324

All that lives will press palms together in prayerful adoration of those who refuse to slaughter and savor meat. Tirukural 260

What is virtuous conduct? It is never destroting life, for killing leads to every other sin. Tirukural 312, 321

Goodness is never one with the minds of these two: one who wields a weapon and one who feasts on a creature's flesh. Tirukural 253