Type 1 is increasingly seen as an auto immune disorder which is likely triggered by those with a certain genetic make up. It's not just one gene. The auto immune system attacks the insulin producing beta cells as it sees them as 'foreign', rather like it attacks other foreign bodies such as bacteria. Of course, this group can include obese people but it is by no means exclusive to them.
Type 2 is seen as a metabolic disorder where the insulin producing function is impaired. There are likely several causes, obesity being one major cause though most obese people are not diabetic. Again, why some obese people get type 2 whilst most don't probably has some genetic explanation, though it is not well researched. Everyone it seems, has their own particular tipping point, from non diabetic to diabetic, but no one knows what it is until it happens. A very good example of life's a lottery. If obese type 2s can feel cheesed off, spare a thought for those type 2s who are not obese. Some rather thin people can get type 2. That's like someone else holding the gun, spinning the chamber and playing russian roulette with your life. At least, if you are eating too many doughnuts, you know you shouldn't be and that you run a risk.
The fat cells that you have are laid down at birth. Your body expands or contracts its fat content in these locations. That's why weight goes on and off certain parts of the body and no matter what they do, some people just can't lose it here or there. Some poor souls just get it in the wrong place, liver and pancreas, from the word go.
Fat cell number is set in childhood and stays constant in adulthood