There is no one single reason why people get diabetes Type 1 or 2. Multiple factors play a role and what causes Type 1 diabetes differs from what causes Type 2.
In diabetes Type 1, the cells that produce insulin (beta cells in the pancreas) are destroyed by your own immune system. Diabetes Type 1 is an auto-immune disease. In the development of an auto-immune disease like diabetes Type 1, genetic predisposition plays a role.
It seems multiple factors play a role when developing diabetes Type 1, like the amount of sunlight and vitamin D, infections experienced during childhood, the barrier function of the intestines, breastfeeding or bottle feeding, and chemical influences. What their role is exactly, is not yet clear. A lot of diabetes research is being conducted to find out the answers.
In diabetes Type 2, your body is less sensitive to insulin. When this insensitivity occurs, the pancreas will at first produce more insulin, therefore you do not yet have symptoms. But after a while, the pancreas becomes fatigued and produces lower and lower amounts of insulin. That's when diabetes Type 2 symptoms appear. Genetic predisposition plays a big part, like being overweight. When you are overweight the composition of fats in your body changes. The ‘bad’ fats pile up in your organs and tissues, causing the sensitivity to insulin to decrease even further. If you are overweight, not exercising enough, and have unhealthy eating habits, you have a higher chance of developing diabetes Type 2.