Some of the above answers are incorrect.

Type I - genetic, usually triggered by an outside virus - your own white blood cells attack healthy, insulin-producing beta cells, thinking they are not YOUR cells (like they are part of the intruding virus), so the pancreas' beta cells no longer produce any (or maybe a TINY bit) insulin because they have been attacked.

Type II - generally, a lack of exercise and a poor diet make it difficult for insulin to enter cells, so the pancreas produces insulin at a normal or perhaps slower rate, and then medicine is helpful for letting the insulin interact with cells to let sugar in.

So - type I is NOT caused by too much sugar, but type II may be!