I believe that obesity attracts massive social stigma ; fuelled by a celebrity-driven culture, society can be quite ruthless when it focuses its sights on fat people. And it comes from all levels of society too ; when they aren't simply labelled as lazy people who eat too much, they are seen as a drain on healthcare resources, with some even suggesting they should pay for their treatment!
Sadly, even amongst a number of doctors, they are viewed as non-compliant when they are unable to lose weight after successive consultations, ignoring the fact that there usually are valid underlying physiological and indeed societal triggers to their condition. The way they are treated in public could also predispose them to mental health issues, which have implications in themselves.
I am not saying the will to change is completely out of their hands (if that is what they want, because surely this society prides itself on 'freedom of choice'), but I think we should collectively be more empathetic and less judgemental in our approaches when dealing with people who are obese.