I'm a vegetarian, mostly. There's no one reason for my decision, but my basic reasoning follows. Concern over the environmental impact of eating meat: it takes a lot of energy to produce meat compared to just eating the stuff that we feed the animals that we eat. Treatment of animals that we eat: while I don't think killing animals to eat them is wrong, the way we treat most animals we raise to eat is horrendous (e.g. factory farms, force feeding, feeding cows corn). Dietary concerns: Humans historically, except in a few rare cares (e.g. Inuit peoples), have not had diets heavy in meat. The amount of meat in the modern diet is crazy, and there seems to be some evidence that reducing it has some health benefits.
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