I think that the idea of loosing weight can also be scary for some people. You get attention, not only from the people you know but also more attention from strangers (who tend to treat fat people as invisible). There can also be that whole "what if I lose weight and then gain it back again" fear of failure. I have a friend who describes some of his self destructive behavior as a "comfortable pain," sort of a devil you know vs. the devil you don't.