This can be explained by the fact that in cats' and dogs' DNA the gene Tas1r2 is absent, responsible for the sensation of sweetness. In this regard, the pet can't control the amount of sugar entering its organsm, which can lead to poisoning.
Constant eating of sweets causes diarrhea, lethargy, refusal to eat, vomiting, stomach upset. Sweet snacks are especially dangerous for aging and chronically ailing pets, kittens and pregnant cats. Even a slight taste of sugar hits into the animal's weakened body can provoke hair loss, itching, blurred vision, diseases of the mouth, aggression or panic, teeth loss and weakening of the immune system.
Also, too many sweets in the feeding causes a thinning of the capillaries, the deterioration of the pancreas, adrenal dysfunction, a significant decrease of protein digestibility by oganism, early cells' death, which subsequently leads to rapid aging. Apart from all this, this diet increases the risk of pancreatitis.
Since cats are predatory animals, protein is useful for them but not carbohydrates which are in large amounts of the sugar. Pets are sedentary, resulting unprofitable carbohydrates store in the organism as fats leading to obesity. Here starts to develop such diseases as diabetes. Cats suffering from this disease are usually required to be appointed a diet with low carbohydrates. Low-carb diet is necessary for life. If the diet of the animal will continue candies, cookies and other sweets, then gradually internal organs of the animal will become fat, which will eventually lead to a violation of their work.
Chocolate is very dangerous for cats. It includes such alkaloid as theobromine, which causes heart palpitations, high blood pressure, and overdose causes intoxication, which often leads to death. To sum up, sweets are not only useless for a cat, but also have detrimental affect on its organism.