The quick answer is, although genetics play a role, diabetes is a lifestyle disease.
Think of sugar as quick energy. It can get converted into the currency your body uses for energy very quickly. But the
flip side is, you can only have so much of it on hand — your blood can only accommodate a few tablespoons of sugar at a time. Your body needs to get rid of any excess quickly, before it causes vascular damage or hyperglycemic symptoms.
So when your body has to get rid of it, where does it go? Basically there are two compartments in your body: intracellular (inside the cells) and extracellular (outside the cells). The bloodstream is outside of the cells, so sugar has to get inside the cells in order to get out of the blood.
Cells are pretty guarded about what they let in and out, though. Very few things can “come in unannounced” – they have to have a friend vouch for them at the door. The friend that vouches for sugar is called insulin (produced in the pancreas). But cells can’t store sugar in its present form, so as soon as sugar comes into the cell, it gets converted from “quick” energy into “potential” energy — fat.
Problems come in when this cycle is repeated too often, too long: like a drug addict needing a bigger dose for the same high, your body will start to require more and more insulin to keep up with your sugar intake. Eventually, your pancreas can’t keep up with the demand. This leads to Insulin Resistance and Diabetes.