I believe the theory is, is that the body uses insulin to convert glucose to glycogen, AKA 'animal starch', and every other effect (that I'm aware of) it has on the body is not considered good. Example: It promotes triglyceride production from fatty acids (high triglycerides are usually a big factor in heart disease), and decreases the metabolism of fats (I think everyone wants this one high), proteins and reduces gluconeogenesis, which can exacerbate lactose intolerance in some people.
Everything more complex than raw glucose doesn't require insulin. That's why the body largely doesn't react to fructose (which is a result of our ancestry, primates largely live in a symbiosis with the trees they feed off because larger animals like Elephants frequently kill the trees in the process of foraging, but shrieking monkeys tend to stop them) so that we do consume more of it. I think it also helps that foods high in fructose (IE fruits) are generally also high in vitamins. I mean some of the recommended RDA's for vitamin C are actually around 6,000-12,000 mg a day (30,000 mg if you're sick). Basically from what I understand, if you look at our close relatives diets (which are very similar to our ancestors diets) they essentially eat 1:1 ratio of vitamin C (in mg) to kcals. So an adult male should eat 2,500 mg of vitamin C and 2,500 kcals. Incidentally vitamin C is used in the body to produce carnitine, which moves fatty acids into the mitochondria to produce ATP.
Your average goat produces around 13,000 mg of vitamin C in normal health through biosynthesis, which has to make you wonder about WHO's RDA of 45 mg a day if a goat produces 290 times that amount. We, unfortunately don't biosynthesise vitamin C.