Animals that live in nature constantly struggle to get their necessary amount of food resources. They have to scavenge for their daily intake of energy, necessary to survive. They are programmed by evolution to be the most efficient in getting energy into their bodies. The types of food that have the highest amounts of energy in them, are sugars and fats. That is the reason why animals (and humans as well) like the taste of it. It gives them a higher chance on survival.
However, when they are taken care of by humans, they do not need to hunt anymore, so they have to use less energy and get no exercise. At the same time, their diet chances from fruit and plants to a mixture of fruits, plants, meat, sugar and protein rich, types of food. Sometimes they find this food inside and around the house by accident, but sometimes the owners of animals, or visitors, want to be nice to an animal and give them some extra food. Humans do this in order to get liked by an animal, to get an animal to move into a certain direction, or just to get rid of food that they don't like or cannot finish. The result is that an animal eats way more than it needs as a daily intake, and does not need to exercise for it. Humans are mainly motivated to exercise by social pressure and thin role models.