In natural situations, many animals tend to prefer vegetation over meat. It is healthier. Proven by evolution.
The reason why we can eat meat is because we couldn't find enough vegetation to feed our needs, so we enabled ourselves to eat meat when vegetation was not possible. Although a "natural diet" may involve consuming both, that diet isn't necessary the best one a human could have. Evolution can't sort out the problem that we couldn't collect enough vegetation. Many studies have shown that humans live longer on a vegetarian diet.
It we were all vegetarian, we would also help sort out the world food supply problem (and help increase oxygen levels while we're there), since vegetation is better production per square unit of land.