Why do glucose levels increase?

The food you consume gets converted into glucose and is absorbed from the intestine into the blood. Insulin, a hormone produced by beta cells of pancreas, is required to pump the blood glucose into the body cells and to convert sugar and other food into energy that is needed for daily activities.

In diabetes, either the pancreas fail to produce adequate amount of insulin or body uses insulin produced by the pancreas ineffectively. Therefore glucose cannot move from blood into the cells and thus the blood glucose levels increase.