To my mind, insulin lies at the heart of the problem.
We know, for example, that all overweight people exhibit elevated insulin levels. Insulin is an anabolic, fat building hormone, as Lanie said. It's easy to see how elevated insulin promotes weight gain therefore.
Type 1's have a chronic lack of insulin and are typicaly slim. Type 2's overproduce insulin at first which promotes weight gain and insulin resistance, and often struggle with excess weight. Again, it's insulin.
It goes further than that though. When insulin levels are elevated, the body cannot access its fat stores for fuel. The fat is locked up, inaccessible, and that lack of fuel promotes hunger for more carbohydrates as a fuel for the body cells instead.
Only when insulin levels fall is it possible to lose weight and keep it off. This can be done by calorie resriction of course, but hunger inevitably follows calorie restriction. The only sensible and sustainable method of weight loss must be carb restriction therefore.
This way, insulin levels remain low, the body can access fat stores for fuel, and hunger is much less of a problem.