Originally it might have been better for humans to eat meat to survive but by now our bodies have evolved to be able to live off of other food sources. All Homo Sapiens are and always have been omnivores which means we can much eat meat, grains, fruits, and vegetables. As our resources grow and science becomes more advanced people figured out what humans need to survive (like specific nutritional facts including how much calcium, protein, vitamins, fat... humans need to live a healthy life.) I find that I can live easily as a ovo-vegetarian. I mean, yes I have protein powder every night and I have to take calcium chews but I feel its so much better than having animals die for my sake.
Also, milk is far from natural. We are the only animals to drink milk after babyhood. I find it quite creepy that we drink the food that is meant for babies on a daily basis.
Even if it wasnt healthy to be vegetarian i still think i would be one because of how cruel animals are treated these days. Humans demand to much meat that we have forced animals into tiny stalls and shoved steroids into them so we can produce tons of meat. It is disgusting how animals are treated.