For many Hindus, vegetarianism is more than a way of life, it is a tradition. Vegetarianism can also be realized as a daily sadhana, or spiritual practice, for a vegetarian lifestyle becomes a practice of ahimsa or non-violence in thought, speech and action.
Vegetarianism Not a Must for Hindus
Though vegetarianism is associated with Hinduism, it is not true that even a majority of Hindus are vegetarian, nor is it a condition of Hinduism to be a vegetarian. In fact, it is stated in the Manusmriti (5:56), There is no sin in eating meat but abstention brings great rewards. So, one cannot state, I am a vegetarian because I am a Hindu. Stating this may also imply that all Hindus are vegetarian, which is not true.
In this case, why are some Hindus vegetarian and some are not? It is believed that before India was invaded by outsiders (Vedic era) the majority of Indians were not meat eaters. Influences of outsiders (i.e., Aryans and Muslim invaders) came to change that. It was also common practice for the Kshatriya caste (warriors) to eat meat as it gave more strength and set the mind up more for fighting. This may seem odd, after all how can eating meat make one feel more predisposed to violence (i.e., anger, aggression, fighting moods, etc.). This stems from the philosophy of himsa or violence.
Is Meat-Eating Violence?
By partaking in eating meat, especially in the old days before prepackaged, supermarket foods, and fast food, one had to think of where to find the animal, how to kill it, how to prepare it for consumption, and then how to cook, eat and preserve it. Hence, the whole process of eating animals was himsa, because one had to think of all this, possibly speak of it (planning the killing, etc.) and act on it by killing, preparing and eating the animal. In Indian history, we have the seminal example King Ashoka (circa 273-232 BC), who - from being a ruthless warrior - not only became a Buddhist, but also promoted ahisma and vegetarianism in his later life.
When we eat the flesh of a dead animal, we not only partake in himsa in our own spirit, but we can also become affected by the spirit of the dead animal. In order to have been eaten the animal had to die. In dying, it felt pain, it struggled, cried, tried to continue living as long as possible. Since it was slaughtered, it died in fright, pain, mental and emotional anguish and struggle. Then it has to be skinned, gutted, processed and packaged to end up on a plate, decorated and consumed in human pleasure. In human pleasure, one does not think of the pain of the once living animals on the plate. Hence partaking in eating meat, one is not just ingesting and digesting protein and nutrients, but the feelings of violence which erupted in the animal from its unnatural death.
The Mahabharata states: "The purchaser of flesh performs himsa by his wealth; he who eats flesh does so by enjoying its taste; the killer does himsa 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 of 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." (Mahabharata, 115:40)
Similarly, vegetarianism and ahimsa can be realized through the old adage treat others as you would like to be treated. Swami Dayananda, in his The Value of Values connects this to ahimsa and vegetarianism by stating that we should not think of somebody being our dinner if we do not want to be somebody elses dinner.
Further, the Mahabharata (18.113.8) says: One should never do that to another which one regards as injurious to one's own self. This, in brief, is the rule of dharma. Yielding to desire and acting differently, one becomes guilty of adharma.