Changes can occur in the retina in the eye, eventually leading to blindness. The kidneys can be affected and eventually become unable to filter waste products from the blood. The nerves might be affected and a person might develop numbness, tingling, or other sensations or might lose sensation all together, leading to an increased likelihood of injury because pain is not perceived. There are several risk factors for developing diabetes. Many of these have to do with lifestyle.
Obesity is clearly a risk factor.
It therefore takes more insulin to keep the blood sugar under control. This is referred to as insulin resistance. Sedentary lifestyle is another risk factor. By not exercising, a person tends to lose muscle tissue and gain fat tissue. As above, this leads to insulin resistance.