2. Food Allergies or Hypersensitivities
This is what's behind the ''betcha-can't-eat-just-one'' syndrome. The very foods that we are sensitive to produce a response in the body that is followed by the release of endorphins, the body's own natural painkillers. Those endorphins make you feel good, and it's easy to become addicted to foods that do that. They're like cigarettes to a smoker. Cigarettes aren't good for the body, and the first time you smoke one you choke. But once you adapt to the ''damage,'' you're hooked. And if you quit smoking and start again, what happens? You crave them all the time. And the foods that cause the most problems are usually the same foods that are most tempting during outings, vacations and other events.

3. Brain Chemistry
Our desires for food are strongly influenced by neurotransmitters such as serotonin. When serotonin levels are depressed or depleted for various biological reasons, we're subject to cravings (the candy craving many women experience during PMS is a prime example). Stress plays a part too, as high levels of cortisol (a stress hormone) can cause us to crave carbohydrates.

4. Genetic Factors
Although we're far from having a complete understanding of this one, there is virtually no one on the planet who doesn't believe that lurking in the genome are at least some genes that influence appetite and weight gain. There is undoubtedly a genetic component that makes it easier for certain people to put weight on and harder for those same people to lose it. There may be a genetically determined weight range that your body ''prefers.'' And although some people can indeed get out of that range, don't expect to do it without a bit of resistance from the universe.