There's been quite a bit in the U.S. written on why and how we get fat. Just go to any bookstore and there will be a whole section on diets, usually found nearby, but separated from the cookbook section. Below is a chart that I think is helpful in explaining the how and why our bodies get fat. Click this link if the chart is too small for you to read as I wasn't able to make this one any larger.
If you know someone who blames their weight gain on a hormone imbalance and you've been inclined to doubt them, then doubt no more. We don't often think of insulin as a hormone, but that is exactly what it is, and it turns out that it is the key to most weight gain.