We humans evolved when food was scarce and life was full of arduous physical activity. Hence, our bodies instruct us to eat everything we can lay our hands on and to exert ourselves as little as possible.
That's right. We are, in essence, hardwired to be lazy overeaters.
This was a perfect strategy for success thousands of years ago. No human could survive in 40,000 BC unless he or she ate anytime food was available. Our ancestors knew that famine was always close at hand-feast now or suffer tomorrow. They were also careful to expend as little energy as possible, because burning more calories than absolutely necessary was a threat to survival. There were unpredictable intervals of low food intake, even occasional starvation, interwoven with times of abundance.
In the modern world, a hunter-gatherer would follow those same principles: He'd eat a lot and move a little. And he would suffer the same ills we do, living in a modern environment where food is abundant and physical activity is more or less voluntary. Most diet and exercise plans ask us to move more and eat less-a direct contradiction of our genetically engineered impulses. No wonder most diets don't work.