There are many reasons for this.  My guess is a lack of knowledge & in a lot of cases depression in combination with improper diet & exercise.  I have lost over 55 lbs & am below 200 lbs. for the first time in probably 5 years at least.  In my case a lot of it was depression & I swear food is like a drug to me.  At night all I want to do is eat.  The only way I've been able to tame that is w/ appetite suppressants.  I diet & exercise now, but when I was depressed I just wanted to lay around.  I'm sure that lack of motivation adds to obesity too.  I know it took being embarrassed by a photograph taken of me to get me off my butt.  My sister shared this & other photos with a group of family members.  It was not done to hurt me, but I was deeply embarrassed.  I dropped my kids off at school & drove home, crying.  I have given up beef, coffee, soda, bacon, sausage, and drink nothing but water & skim milk.  I limit myself to a bottle of wine every 20 lbs. lost.  I go to a medical weight loss facility on a weekly basis to get weighed.  This is working great for me.  No, I haven't had any surgery.  I ride my bike, rollerskate, or workout at the gym.  I believe that I suffered from post partum depression after my youngest was born (@ 6 yrs ago) & never got treated for it.  I'm just thankful that I didn't do anything I'd regret (besides laziness & overeating!).  Overweight people need encouragement & support...not insults & stares.  When people are insensitive to their feelings, it will not push them to lose weight.  I'm sure for most, it will just depress them more.  There are many reasons for obesity & there is not a right or wrong answer.  Other things that I think can contribute to this problem are: high consumption of trans fats & saturated fats, high calorie intake/not enough calorie burn, not enough water intake, too much sodium in diet, too much soda, childhood trama w/o proper counciling...