Too much of a good thing
I doubt that high fructose corn syrup in and of itself is the cause of any health problem. After all, fructose is a naturally occuring sugar, it's the main component of the sugars in various fruits.
The real problem is that the human body evolved over the millenia as a hunter-gatherer. This meant a diet that was high in fiber, low in fat, low in carbs, low in sugars, low in salt, relatively low in protein. As a result, the human appetite has a natural affinity for these scarce dietary components - we crave them. As humans developed agriculture, they started focusing on crops that they could store for at least a year - cereals and grains, and extracted oils, foods preserved with salt, etc. So the diet shifted to high carb, higher fat, higher salt. The domestication of animals meant that protein was more readily available. The human body had adapted to low amounts of these things because they were generally scarce. Nowadays, most people can eat high fat, high protein, high carb, high salt diets, and many do, because they let their natural dietary instincts direct their food selection and amounts. If there is a problem with high fructose corn syrup, is that it simply makes vast overdoses of sugars available for consumption at very low cost and without making one feel satiated.
We are the victims of our own success. Our abundance can kill us.