How does diabetes develop?

Genetic / lifestyle: It can be a genetic condition over which you do not have control. You can also decrease your risk of type 2 diabetes by eating a healthy diet low in simple sugars, exercising regularly, and maintaining a healthy weight.

Bad diet : Adult onset diabetes is very preventable. Excessive sugar and carbohydrate intake cause high blood sugar levels, which then make your Insulin levels rise. Over time the chronically high Insulin levels lead to Insulin resistance and you will need to make higher and higher levels of Insulin to maintain normal blood sugar levels. Eventually you will have full blown diabetes.

Broad question: Thats a broad question. Type 1 diabetes is an autoimmune disease. It is only 30% genetic. Type 2 diabetes is 80% genetic. It starts off with insulin resistance. The body just makes more insulin initially but eventually it cant keep up and the blood sugars start to rise. With a genetic predisposition if one eats processed sugars, becomes overweight and does not exercise type 2 diabetes may set in.