With most people, it is not a single thing that is causing their insulin insensitivity. Some people develop diabetes at a young age and are insulin dependent due to the destruction of the insulin producing cells of their pancreas (this is known as type I diabetes). More commonly, it is a lifetime of exposure to high levels of glucose (sugar) that cause the pancreas to "wear out," which is known as type II diabetes.
The pancreas makes insulin in response to elevated blood sugar in normal, healthy people. If it has to make too much for too long, it seems to have difficulty keeping up. The body responds with a somewhat inflammatory reaction in the pancreas that causes the insulin producing cells to be replaced with inflammatory tissue, reducing their ability to respond. Additionally, the rest of the cells in the body of these people become resistant to the high levels of insulin that they are used to seeing, meaning that it takes more and more for the same effect (think of your morning dose of coffee).
Diet and exercise can make the body respond more fully to the changes in blood sugar levels for these people. Please work with your doctor
to optimize your diabetic control and reduce the complications it can bring.