First and foremost, we have to realize that our bodies have a set point of weight that they like to maintain. If you've weighed 240 pounds for a while, your body will do everything in it's power to keep you at 240 pounds.
They have done studies on inmates in prison where they have overfed half of them for six months, and underfed half of them for six months. Each group gained and lost weight respectively during the six month period. Then they let them go and allowed them to eat whatever they want. Both groups reverted back to their original weight. The group that lost weight, gained it back. The group that gained weight, lost weight down to their original weight.
Further prison inmate studies demonstrated that you have to maintain your new weight for at least 9-12 months before your body resets to this new set point and starts trying to keep you at this new weight.
So if you weigh 240 pounds, and lose 50 pounds. You must stay at 190 pounds for about a year, before your body views this as your new set weight, and tries to keep you at this weight.
This is why people have trouble maintaining a new weight. Our bodies try to do everything they can to "protect" us from episodes of feast or famine. Over the years, our bodies have evolved these protective mechanisms. In order to break this cycle, you have to maintain a new weight for approximately 9-12 months. Then your body will reset.