Fructose, also known as fruit sugar, is a simple monosaccharide with the formula C6H12O6. It acts differently in the body than glucose. When you consume glucose, it is broken down by glycolysis, which occurs in many different kinds of cells throughout the body. Because glucose is so important to cell metabolism, we have mechanisms that work to keep the amount of glucose in the blood stable at all times. Insulin is not necessary to metabolize glucose; its function to to keep blood glucose levels from spiking excessively after a meal.
By contrast, fructolysis, the breakdown of fructose, occurs almost exclusively in the liver, where the process is used to replenish stores of glycogen. This process is not quite as critical, so is not so closely regulated by the body. Also, in a natural diet it would be uncommon for a person to have an overload of fructose in the body, because fructose, while common in fruits, is not absorbed efficiently from the intestines. Because of this, we never evolved a mechanism to keep fructose levels under control, as we did with glucose and insulin. However in the modern world, fructose is added to many foods in large quantities, and there is some research that indicates that ingesting large quantities of fructose over time may increase the likelihood of developing Type II diabetes.