This post discusses the rationale behind vegetarianism based on the observations of Sri Aurobindo and the Mother. On the question of diet, there is no generic answer possible in light of the fact that the world consists of people with varying genetic makeup who exist in different stages of evolution – most are inclined to worldly success while some are inclined to inner growth.
The primary reason meat-eating was discouraged for Yoga practitioners is because it makes the subtle(astral) body heavy. Those who are a little advanced on the spiritual path tend to become sensitive to the presence of meat within the body. The animality which invades you with the consumption of meat contaminates the luminosity of consciousness which was established through Yoga. Progress in meditation beyond a certain point is not possible without giving up meat. This is how Ramana Maharshi explains it:
Question: What about diet?
Ramana Maharshi: Food affects the mind. For the practice of any kind of yoga, vegetarianism is absolutely necessary since it makes the mind more sattvic [pure and harmonious].
Question: Could one receive spiritual illumination while eating flesh foods?
Ramana Maharshi: Yes, but abandon them gradually and accustom yourself to sattvic foods. However, once you have attained illumination it will make less difference what you eat, as, on a great fire, it is immaterial what fuel is added. 
For worldly men who live a life dedicated solely to material success, it makes little difference whether meat is eaten or renounced because they are not yet sensitive of their subtle body. And for those engaged in physical labor, meat can be in fact be beneficial because it provides the energy needed for muscular activity, although here one must point out that there is a wrestling tradition in India which relies solely on vegetarian foods for bodybuilding. (See Pehlwani-Diet.) People living in harsher and colder climates may require meat due to their genetic makeup. The best example of this problem is the 14th Dalai Lama who contracted hepatitis when he suddenly gave up meat.
Reader’s Digest: Your assistant says you are half vegetarian. How can one be “half vegetarian?”
Dalai Lama: [Laughs.] In the early 1960s, I became a vegetarian, and for almost two years I remained a strict vegetarian. But then I developed hepatitis, and I returned to my previous diet; for a while it would be vegetarian one day, nonvegetarian the next. My kitchen is now totally vegetarian. But that doesn’t mean I am completely vegetarian, for when I visit places, occasionally I take nonvegetarian…that seems to help reduce the size of my stomach. 
One objection to meat-eating is that one is participating in the slaughter of life. But for most worldly men this does not make much of a difference because they already have a heavy burden of other imperfections and impurities (ego, anger, lust, depression) which have to be shed in the upward spiral of evolution. Certainly, the awakening of the perception to switch to a vegetarian diet as part of an inner purification process is helpful but it is not a prerequisite to the Godward turn. The decision to give up meat must emerge out of conscious growth rather than nervous repulsion or social pressure.
This brings us to the other reason people normally object to meat consumption and that is because they are emotionally repelled and overcome with pity at the sight of animal flesh. This according to Sri Aurobindo runs counter to the principle of equanimity which is the basis of Integral Yoga.
This world is full of so many things which are horrible and terrible. Equanimity means that one should be able to look at them from a certain poise without being perplexed or moved. It does not mean that one will go on killing animals indiscriminately or out of a personal motive. That would be untruth. But it means that one must be able to look at things without being moved. What X calls “pity” is something quite different from “compassion” and both are different from equanimity. Equanimity is an attitude of the whole being. Pity and sentimentalism are results of nervous repulsion, some movement in the vital being….True Compassion is something different. It comes from Above. It is a state of sympathy for the suffering of man and the suffering that is on earth and there is an idea of helping it as far as one can, whenever one can in his own way. It is not like pity. It is like the Gods who look upon human suffering from above, unmoved. 
In the following response, the Mother Mirra Alfassa discourses on the reason why meat should be avoided.
Question: What happen if one eats meat?
Answer: Do you want me to tell you a story? I knew a lady, a young Swedish woman, who was doing sadhana; and she was by habit a vegetarian, from both choice and habit. One day she was invited by some friends who gave her chicken for dinner. She did not want to make a fuss, she ate the chicken. But afterwards, during the night suddenly she found herself in a basket with her head between two pieces of wicker-work, shaken, shaken, shaken, and feeling wretched, miserable; and then, after that she found herself head down, feet in the air, and being shaken, shaken, shaken. (Laughter) She felt perfectly miserable; and then all of a sudden, somebody began pulling out things from her body, and that hurt her terribly, and then someone came along with a knife and chopped off her head; and then she woke up. She told me all this; she said she had never had such a frightful nightmare, that she had not thought of anything before going to sleep, that it was just the consciousness of the poor chicken that had entered her, and that she had experienced in her dream all the anguish the poor chicken had suffered when it was carried to the market, sold, its feathers plucked and its neck cut! (Laughter)
That’s what happens! That is to say, in a greater or lesser proportion you swallow along with the meat a little of the consciousness of the animal you eat. It is not very serious, but it is not always very pleasant. And obviously it does not help you in being on the side of man rather than of the beast! It is evident that primitive men, those who were still much closer to the beast than to the spirit, apparently used to eat raw meat, and that gives much more strength than cooked meat. They killed the animal, tore it apart and bit into it, and they were very strong. And moreover, this is why there was in their intestines that little piece, the appendix which in those days was much bigger and served to digest the raw meat. And then man began to cook. He found out that things tasted better that way, and he ate cooked meat and gradually the appendix grew smaller and was no longer of any use at all. So now it is an encumbrance which at times brings on an illness.
This is to tell you that perhaps now it is time to change one’s food and go over to something a little less bestial! It depends absolutely on each one’s state of consciousness. For an ordinary man, living an ordinary life, having ordinary activities, not thinking at all of anything else except earning his living, of keeping himself fit and perhaps taking care of his family, it is good to eat meat, it is all right for him to eat anything at all, whatever agrees with him, whatever does him good.
But if one wishes to pass from this ordinary life to a higher one, the problem begins to become interesting; and if, after having come to a higher life, one tries to prepare oneself for the transformations, then it becomes very important. For there certainly are foods which help the body to become subtle and others which keep it in a state of animality. But it is only at that particular time that this becomes very important, not before; and before reaching that moment, there are many other things to do. Certainly it is better to purify one’s mind and purify one’s vital before thinking of purifying one’s body. For even if you take all possible precautions and live physically taking care not to absorb anything except what will help to subtilise your body, if your mind and vital remain in a state of desire, Inconscience, darkness, passion and all the rest, that won’t be of any use at all. Only, your body will become weak, dislocated from the inner life and one fine day it will fall ill.
One must begin from inside, I have already told you this once. One must begin from above, first purify the higher and then purify the lower. I am not saying that one must indulge in all sorts of degrading things in the body. That’s not what I am telling you. Don’t take it as an advice not to exercise control over your desires! It isn’t that at all. But what I mean is, do not try to be an angel in the body if you are not already just a little of an angel in your mind and vital; for that would dislocate you in a different way from the usual one, but not one that is better. We said the other day that what is most important is to keep the equilibrium. Well, to keep the equilibrium everything must progress at the same time. You must not leave one part of your being in darkness and try to bring the other into light. You must take great care not to leave any corner dark. There you are.
The Mother, Questions and Answers (1954): 23 June 1954
- Nirodbaran. Talks with Sri Aurobindo. vol. 2, p 1004. (remark by Sri Aurobindo dated 3 Jan 1941 is as follows “Of course it (meat) makes the body heavy, I mean the subtle body. The other objection to a meat diet is the taking of conscious life”.
- David Godman. Be as you are, p 204
- Mary Aikins. Interview with the Dalai Lama. Reader’s Digest May 2004. http://www.readersdigest.ca/mag/2004/05/dalai_lama.html
- A.B. Purani. Evening Talks with Sri Aurobindo, Second Series, pp 61-63.