Why is it that eating meat gives us a disgusting feeling?

I'm not talking about meats you regularly eat. For example, if I told you we were going to be eating alligator, porcupine or tiger meat you would most likely be disgusted. If I told you we were going to eat some unknown berries off a tree (unknowingly poisonous) you would eat them. Even eating tree leaves does not give any negative thoughts even though they wouldn't be very tasty.