The reasons why a person is overweight is multifactorial, but one issue that is probably a number one focus is: SUGAR. Sugar is everywhere, in about all processed foods, often in things white (flour, potatoes, breads) and is in places you don't think about (the second ingredient in most peanut butters, after peanuts, is sugar).
Sugar acts like a drug: we crave it, it makes us feel good, we get addicted to it, and it's not good for us.
The food industry knows how sugar affects us, and how we will keep coming back for more. Sugar affects our brains as noted like a drug, and when our system is overloaded with sugar, we store it, and paradoxically still feel hungry even after ingesting large quantities. In this link, which was the source of the brain scan images, the areas of the brain that are affected by sugar are similar to those affected by cocaine. On this webpage are listed many foods that have huge amounts of sugar.
For info on the Sugar Busters diet, click here.
There is a fairly amusing video on youtube (click here) about quitting sugar.
For another video of a TED talk from Dr. Robert Lustig, click here.
The problem with carbs has to do with blood chemistry. Although they may not look like sugar, the simple carbs in our diet like bread and corn are converted into simple sugars when they are digested. The net effect on your metabolism between eating a bagel and a bag of candy is not very different. The sugars stimulate the production of insulin, which causes fat cells to go into storage mode leading to weight gain. Also, fewer calories are available to feed the body, so we still feel hungry.
Often many orthopaedic conditions are made worse by being overweight, and the first step to losing those pounds is to work on getting sugar out of your diet.