As others have mentioned, the cause is insulin resistance, which is why your glucose levels are so high in the first place. Insulin resistance can be purely genetic or purely conditioned, but it is frequently a function of both. If it's genetic, then your body may not produce enough or have dysfunctional insulin receptors. It could also be that your insulin is imperfectly folded during protein synthesis, resulting in a frequently insufficient protein. For conditioning, the body can adapt to constantly high glucose levels. High glucose levels are frequently caused due to poor dietary habits and body composition, including high refined sugar intake, excess abdominal body fat (cytokines released from fat tissue make you inherently insulin resistance), low fiber intake, etc. With constantly increased glucose levels, the body sends signals to secrete more insulin to inject glucose into active cells. However, due to chronically elevated glucose levels, insulin is less effective at eliciting recruitment of glucose transporters. Eventually, your pancreas gives way and can no longer produce sufficient amounts of insulin or your body stops responding well to your natural insulin levels. Exercise is the number one treatment of type II diabetes since it allows for insulin-independent glucose transport, increased calorie expenditure (hopefully leading to weight loss), and better metabolic regulation.
PS: OP, are you a type I or type II diabetic? If you're a type II diabetic, I suggest you come visit us in the health and fitness forums here at Anandtech. We'd be more than happy to help you try to remedy/address the underlying problem.