Your body likes sugar because it's a concentrated source of energy -- something your ancestors needed in immediate and abundant supply. Modern life is less rigorous, yet has greater availability of sugar for the average human being. This puts stress on the body -- particularly the pancreas -- that early humans simply did not experience, and that your body is not equipped to deal with.
Sugar is a simple carbohydrate, a kind of food that breaks down quickly in your digestive tract. This floods your bloodstream with glucose in short order, which your body can break down for energy. This process is normal, and in reasonable amounts is part of what's supposed to happen when you eat.
Your body converts blood glucose to energy through a hormone called insulin. When blood glucose is in your bloodstream, an organ called the pancreas creates and releases insulin into the blood to process the glucose. As with how sugar works, in reasonable amounts, this isn't dangerous -- only when you eat too much and force the pancreas to break down.
Sugar and the Pancreas
If you eat too much sugar at one time, your pancreas will put out the extra insulin without too much trouble. However, it you eat too much sugar frequently, the long-term wear on your pancreas may cause it to "age" more rapidly than the rest of your body. This can ultimately result in pancreatic failure, which is a fancy way of saying "diabetes."
Vicious Cycle Type One
Sugar intake creates a vicious cycle that makes you eat more sugar. When you eat sugar, your bloodstream floods with glucose. This makes your pancreas produce insulin to process it -- but it will produce insulin as if the sugar will remain in your system for several hours. When the sugar rush ends in a few minutes, your blood will have extra insulin. It will respond by craving more sugar to help it deal with the insulin.
As an occasional treat, sugar is fine and even necessary for your diet. However, eating too much sugar can strain your pancreas. Over a long period, it can even make your pancreas fail.