The truth is, it's none of those things. I am just unwilling to indulge people's fantasies on this issue, because I've lived through it having been clinically obese myself in the past.

>I find it supremely hilarious that you refer to the person's own body as an external factor.

It's external in the sense that we have no control over our DNA, which is implicitly being blamed when one speaks of one's own predisposition towards weight gain. It's not that funny.