No one teaches the animal what to eat and what not to eat. He knows instinctively what to eat. Humans also have the same instinct. The way it works is just eat whatever tastes good and that will be healthy for you and non-fattening. That is why a baby tastes everything. So again whatever tastes good, is good for you. This is why you will spit out spoiled milk.
But wait!!! Man has the technology to create rat poison. It tastes like healthy food (good taste) but kills the rat. So man has done that to you. Note-- any food unaltered by man (uncooked) that tastes good, is good for you. So if you are stranded in the woods, that is how you know what to eat (cooking changes that). So man has created fake fruit. It tastes like fruit that exists in nature but it is terrrible for health, addictive and fattening called sweets.
So it is natural to like real sweets called fruit, like a baby does, but they have fooled you just like the rat. But they do not want to kill you. They want you to become obese so you will buy lots of their foods. This is called GOOD BUSINESS. See this site for more on this: Weight Loss Strategies