I don't really think its the fat, just people with low self esteem that don't want to have fun.
They're wondering too much about how they look or appear to other instead of just having fun and doing whatever they want. Insecurity is a bitch, look at yourself, realize you have some flaws, and do what you can to fix em. Don't give up and have the depressed attitude like that's supposed to help, people who know they want, have to put in hard work to get it, simple as that.