Some people stop eating meat. Why?
Certainly a lot of vegetarians are sympathetic with meat animals. Others have various health concerns about meat; some hold water; some don't.
The strongest argument, in my book, is that raising an animal for food requires several times more protein than that animal yields. Some people argue that we can feed the whole world on a vegetarian diet, but not a meat-based diet. People who act on this argument are called "trophic-pyramid vegetarians."
The argument against trophic-pyramid vegetarianism is that a world getting its protein from beans and grain would demand endless fields of nothing but grain and beans. I can't buy it, because we already have that to support the livestock.
I will say that, were the world to adopt an ecologically sensitive food system (call it "permaculture"), it would probably be irresponsible not to eat meat.