Is it essential or somehow profitable to be vegetarian when doing "Metta/Maithree" meditation?
Or Is it essential or somehow profitable to any other form of meditation?
If not why a lot of people seems to be becoming vegetarian some time after they start their path?
Based on Theravada, which is what I'm most familiar with, vegetarianism is not essential for any meditation that I know of. Even so, I've chosen to be a vegetarian. As I understand it, the Mahayana tradition tends to strongly encourage vegetarianism, but even then I'm not certain if they do it specifically to aid meditation practice.
As to it being profitable, it depends on what effects adopting vegetarianism has on that person's state of mind. If being a vegetarian helps your compassion towards beings, then you might consider it profitable. For some people, choosing vegetarianism might result in no improved state of mind, in which case it doesn't matter.
This also answers your third question as to why a lot of people become vegetarian after they start their path. They might have noticed that becoming a vegetarian, for them, improves their personal state of mind. Of course this won't be the only reason, but for some it might be.
Try it, and see what works for you.
When my SO became vegetarian she said she liked animals (e.g. horses and cows and so on) and other living creatures; and said it would be hypocritical to cause them (e.g. to pay a butcher for them) to be killed for her to eat.
I think she felt that being non-vegetarian (i.e. that being a carnivore or predator) would interfere with or cloud her perception of her love for living animals.
I think there's some justification for that in a text like the Dhammapada, for example in the whole chapter on violence, which starts for example with,
All tremble at violence; all fear death. Putting oneself in the place of another, one should not kill nor cause another to kill.
I think that, logically, that justification depends on three assumptions:
Vegetarianism is sila or part of the training in virtue. According to Buddhaghosa, such moral discipline serves two functions - to compose the mind and to aid in the cultivation of non-regret. I'd say it's fairly obvious that if you are on the cushion and worrying about what you take to be a contradiction between your actions and the lovingkindness meditation you are practicing, then yes, you might benefit from vegetarianism.
It's important to understand, however, that eating meat isn't going to "short circuit" your metta meditation. If that were the case, than the whole of Tibetan Buddhism would be marked by deficiencies in lovingkindness! I will say, however, that eating a vegetarian diet does amazing things for your concentration and that these benefits extend well beyond the absence of regret. Like anything else, the proof of the pudding is in the eating, right? Just not a meat pudding in this case. ;-)
One part of your bodily sensations are due to food we take. Light vegetarian food can help concentration and meditation including Metta. This is beneficial but not necessary.
Buddhism is not an extreme ideology and centres around intention. If you do not have the intention of killing and also the situation is not such what you would be tainted by the guilt that an animal was killed on by behalf, then there is nothing wrong to eat non Veg. food. The main thing is keeping your mind pure and conducive to meditate hence if you find yourself uneasy or guilty about eating non Veg. food then it will be beneficial to give up.
But if you are a monk you have no choice but accept what has been offered.
The Buddha clearly and explicitly prohibited vegetarianism for the monastics. This was part of the controversy that culminated in the heretic Devadatta's failed attempt to foment a schism and to murder the Buddha.
There are various possible reasons for this, all speculative: the obligation of monastics to accept alms to enable ordinary people (puthujjana) to acquire merit, many of whom would not have been able to offer non-meat items to the monastics, seems to be the most likely reason; the scarcity of non-meat food items in many areas, which would cause monastics to leave those areas or starve (starvation was a constant problem for the monastics); religious pride and self-righteousness; and the law of karma, which only holds an individual responsible for actions of which he or she is the direct cause.
The Buddha did forbid that any animal should be killed specifically for a monastic, and monastics were not allowed to accept such offerings. Some Mahayana sutras prescribe vegetarianism, however.
