Well, that assumes we're all the same and that's just not the case. Anything from a genetic predisposition, a physical injury (ie knee or back), stomach problems, to side effects from certain pharma-ceuticals (ie anti-biotics) can effect your metabolism, your ability to exercise and hunger levels, all of which can definitely have a large effect on your overall weight.
I'm sure there are other factors affecting weight gain I haven't mentioned that have nothing to do with emotional overeating and a lack of self-control.