For example, there are hunter-gatherer tribes living today that consume a significant amount of sugar (up to 60% of their calories) and don't have the same problems with obesity, diabetes, etc.
I'm no expert but it seems that the human body doesn't handle bodyfat very well. Perhaps all of this negativity around sugar can be explained by the fact that sugar is calorie-dense and calorie-dense foods tend to make people fat. Especially in the hyper-palatable forms we have in America.