Why can't you stop eating at the sight of delicious fatty or sugary food, even while your stomach is full? Answer: You have a good system in place to avoid over-eating, but it's overridden by your caveman instincts. Confused? Watch this video.
When your stomach is empty, it produces an hormone called ghrelin, which interacts with a neurotransmitter in your brain called NPY. This turns on your hunger. When the stomach is full, it produces another substance, called leptin. Leptin has the contrary effect, and NPY turns off hunger.
But there's something else: the sight of fatty and sweet food, high in energy, fires up a prehistoric instinct. Our ancestors were always on the prowl for food in a world where that food was hard to catch. They had to save every bit of power to survive and, while presented with the opportunity, eat and store it. Just like any other animal on Earth.
And here's where things get interesting: since this kind of food is everywhere, we indulge ourselves, entering a vicious circle that interferes with the ghrelin-leptin mechanism, turning us into fat people. Unless you exercise, that is.