The basic theory is that humans never evolved to handle the sheer amount (at least, a lot of us can't) of simple carbs and sugars that modern life makes available to us via a “typical” diet. The sheer amount of insulin that is needed to handle that much sugar in the bloodstream is constantly pumping, the body builds up a tolerance to it, which in turn stresses the pancreas. That can lead to pre-diabetes and diabetes, which is one of the most common chronic diseases, at least in the USA. Also leads to weight gain as a primary side effect.