>>1
I know I'm feeding the troll, but, many things.
Not enough exercise, too much junk food. I know someone who drinks a lot of Monster and Redbull energy drinks, which is bad for you, too much processed and not enough raw and pure food, ect.
Shii offers His explanation.
http://www.shii.org/knows/Obesity
ALso, don't begin your thread title with "Why are americans so fat and disgusting???" it makes people think you're a troll, even if you know yourself that you are not.