Lifestyle choices , ethnicity and genetics cause T2 diabetes (not sugar or fructose).
It all starts with insulin resistance (IR), which occurs well before T2 diabetes is diagnosed. This is when the body cells do not respond that well to insulin, to let the glucose go from the blood stream into the body cells , so the glucose can be used as energy. This causes the blood glucose to rise. Other metabolic changes also occur one of which is a higher amount of insulin being secreted in response to the higher blood glucose levels but IR is there already.
While we know some information about insulin resistance we do not completely know it all. People have insulin resistance well before T2 diabetes is diagnosed. Some ethnic groups are genetically IR, some medications cause this. What we do know is that some lifestyle choices cause IR, those that cause obesity especially central obesity, related to eating too much processed carbohydrates (as Rebecca has said), saturated fats, cigarette smoking and inactivity. Some research suggests that IR may be present in some genetically predetermined slim people but the blood glucose levels only rise in an environment of inactivity, poor diet and high central obesity as visceral fat causes hormones which cause IR.
The role of fructose is very fashionable and controversial , as most of the studies are done in rats with very high fructose concentrations, well above what humans consume. So while fructose has been shown to cause IR in the rat we cannot translate that into humans. What we do know is sugar sweetened beverages being a processed carbohydrate drink , having both fructose and glucose should be avoided as this causes central obesity. Humans very rarely eat fructose in isolation ( except in the lab) as sugar is both fructose and glucose or at the levels used in the rat model. We also know for some, fructose is poorly metabolised and passed out into the toilet before it even gets to the liver. Liver enzymes are raised with IR especially ALT but it is always related to many lifestyle choices and metabolic factors and really you can't put this done to one thing only.
Exercise improves IR. Studies show that after exercising, muscles become more sensitive to insulin, reversing IR and lowering blood glucose levels. Exercise also helps muscles absorb more glucose without the need for insulin. The more muscle a body has, the more glucose it can burn to control blood glucose levels.
Choose a Mediterranean type diet that has minimal processed foods, moderate amounts of monounsaturated fats, more plant based foods that are more whole.