How does diabetes occur?

Genetic propensity. Combined with obesity (which means you need more Insulin to get sugar into cells) 'stressing' beta-cells that produce insulin. If one's beta cells can't keep up with the increased Insulin demand, sugar doesn't get into cells, blood sugar rises, = diabetes, type 2 in type 1 something we don't understand, in environment, leads to destruction of Insulin producing cells.
High blood sugars. Diabetes is a serious disease in which a person cannot make any or enough Insulin to transfer blood sugar into the body's cells. The blood sugar stays in the blood, and starts to leak through the kidneys, into the urine. Meanwhile, without more insulin, the cells in the body can't bring in enough blood sugar to use as fuel. Diabetes damages cells and tissues throughout the body.