http://www.gnolls.org/ http://www.gnolls.org/2304/why-are-we-hungry-part-1-what-is-hunger-liking-vs-wanting-satiation-vs-satiety/ http://www.gnolls.org/2320/why-are-we-hungry-part-2-hunger-is-the-product-of-multiple-perceptions-and-motivations-sometimes-conflicting/ http://www.gnolls.org/2342/restrained-eating-willpower-and-why-diets-fail-why-are-we-hungry-part-3/

Conclusion: A Successful Diet Must Minimize The Role Of Willpower

The problem here should be obvious:

Restrained eating causes stress. Continually exercising your willpower in order to eat less food than you want—or different food than you want—is stressful.
Stress makes you eat more.

Full citations. The series is worth reading.