The first reason is you cannot oxidize fructose or blood glucose any longer like the average person from only eating large portions of meat/protein or only eating meat and not enough vegetation long-term?
The second reason, the fiber in the fruit takes longer to process so the fructose is not absorbed as fast as eating a free sugar that shock your insulin secretions into an enormous chronic spike?
Why Whole Foods are better!
Third reason, your more intolerant to the fructose because you do not eat enough fruit to to give your body enough hormetic shock to withstand inflammation of what little sugar your giving it.
The last reason, your body is now dependent on the vicious circle of storing carbohydrates rather than oxidizing both fat and carbohydrates. (hint: your only oxidizing protein into sugar/glucose)
Sugar is not technically a bad thing until you start eating more than your muscle tissue (without fatty deposits?) can burn at one time. The less lean skeletal muscle you have in ratio to adipose tissue size, the less sugar you eat or can eat.
Fatty liver and/or pancreas (eat more eggs) then you will need more insulin when you over eat sugar or any amount or type of sugar.