Sugar is concentrated. Normally you find it in the sugar cane or in the sugar beet. But not concentrated. People do not eat small amounts and balance it with other foods that contain protein and healthy fats. So the sugar overdose in the diet leads to an insulin response that causes inflammation in the body. This leads to hardening of the arteries, to high blood pressure and the excess triglycerides lead to storage of fat (obesity). After a number of years of further disbalanced food intake with too much sugar the person may come down with diabetes.
So, enjoy small amounts of sugar, preferably in its natural context (in fruit) or in natural yogurt mixed with fruit. Otherwise refined sugar can be bad for you.
OMG, I am so impressed with you comment <3
I eat little fruit and a little sugar once in awhile in my baked goods. However most foods eaten today are processed fast foods, not whole foods. The book I linked to called Salt Sugar and Fat goes into how the food industry has hooked people into eating food that is empty of nutrition. https://www.npr.org/sections/thesalt/2013/02/26/172969363/how-the-food-industry-manipulates-taste-buds-with-salt-sugar-fat