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.
However, animals don’t have this mental pressure, and will just sit back and enjoy the food to the max. Their body is programmed to preserve as much energy as possible, and transform it into body fat. This, so they would have reserves in worse times, such as winter. However, reserves are unnecessary when animals live around humans. They have a constant supply of food, easy access to it, and even when they are missing a meal every now and then, when they get skinny, pet animals usually get a larger portion of food for a while. Hardly any pet animal dies of being underfed. However, an increasing amount of animals are dying of obese related causes. Fat animals have a much shorter life span than fit and exercising animals. They lack having predators hunting on them like they would when living in nature, but dangers to their lives exist in the form of excess. Animals getting fat only serves them to look cute, or more alike their owner indeed.