According to my research the causes of diabetes are not clearly known. There is still a mystery in it.
Some says diabetes is inherited and runs in the blood so if the history of your family suffer from these kind of illness it might have the tendency that you can have it. But I think diet is also one of the major factor and responsible for causing diabetes and may be we can control it by watching over our diet. My grand mother died due diabetes. Some of my auntie diagnosed with these kind of illness too. I'm afraid someday I can also suffer from these kind of thing runs into my blood.