Diet Distortions: "Eat only when you're hungry."
If our hunger were calibrated so that we were satisfied with the exact amount needed to maintain a healthy weight and no more, then this "rule" might work.
Unfortunately, the evidence is quite to the contrary. People differ in their degrees of hunger, and those with a tendency toward greater weight gain tend to be hungrier. This seems in part to reflect genetic differences in appetites. Of course, people sometimes eat when not hungry, and there is evidence that those who do this more seem more susceptible to weight gain.
BOTTOM LINE: It's not fair and it's not easy, but for some of us, the solution to excess weight gain may mean occasionally eating less than fully satisfies our hunger.