It's all well and good to talk about having parents who didn't do well by me in a nutritional sense when I was a child, but I'm an adult now and responsible for my own choices, food purchases, preparation and eating habits. This is all true. Of course, it's not so simple. If things were as simple as fat bashers pretend they are, there wouldn't be an obesity problem
The problem for people who grew up as fat kids and are presently fat adults is that they can't simply put on the breaks of their lifestyle and biological condition up to this point and jam it into reverse. It's not only the psychological issues that are at play, but a lot of biological ones. Children who grow to adulthood overweight are essentially going to live their lives in a house with a poorly built foundation. No matter how many times they try to make improvements or patch things up, it'll never be as strong, resilient, or comfortable as a house which started out on a sound foundation.
For instance, the number of fat cells in your body cannot be changed once you reach a certain level of maturity. If you are thin upon maturing, you won't get more fat cells. If you're fat, you can't reduce the number unless you have surgery. The total number of fat cells affects how your body metabolizes and deals with food as well as affects hormone balances. Someone with more fat cells will have a propensity for storing fat more readily than someone with fewer. They also cannot change the disposition of their body's shape and size because the cells can only be shrunk or expanded, not eliminated (without lipsuction). This is why people have "problem areas" which they can exercise and exercise to improve but never get rid of. Those cells aren't going anywhere past a certain age.
What is more, estrogen, which is stored in fatty tissues has an effect on insulin and insulin resistance. More body fat will ultimately affect how you metabolize food and how fat is stored. Insulin resistance will ultimately affect appetite and how often you feel compelled to eat and what you want to eat.
I don't want to go into a bunch of biological mumbo jumbo as an "excuse" for fat people staying fat because I don't want to provide excuses. I want to talk about reasons. One of the reasons it's harder for a fat person to lose weight is that their body is different than a thin person's. They're fighting a much harder battle and that battle never ends because the "house" that their parents built for them isn't very sound and it's never going to be as sound as it could have been. Things that a thin person takes for granted, like the ability to resist an urge to eat a piece of cake, are a much harder task for a fat person. Imagine the difference between resisting an ice cold drink of water when you are parched and haven't had a drink for days as compared to doing so when you just have slightly dry mouth.
A fat person's body is always saying, "I'm parched", even when they're not. It is very hard not to give into the biological and psychological pressure to eat when that pressure is intense. Unless you grew up fat and are a fat adult, it's difficult to know just how tough it is to resist at times. Thin people aren't judging "willpower" from the same vantage point as a fat person. We've got a megaphone screaming "eat" at us and thin people just have a nagging whisper. One is a lot harder to brush aside than the other.
That isn't to say that megaphone can't be ignored, and one can condition oneself to do it (and it gets easier the more you do it), but just judging fat people and offering pat advice without considering the complexity of the problem is ignorant and pointless. Unless an enlightened attitude toward the problem starts to spread, it'll never get better. The solutions for a thin person who gains weight won't necessarily work for people with a lifetime of weight problems.
