Most people identify with this: we eat one potato chip and before we know it the whole bag is gone–even on a full stomach! German researchers asked the question why? They concluded by mapping rat brains that this and many other kinds of food stimulate the reward center of the brain. It is called "hedonic hyperphagia" or simply eating to excess for pleasure, rather than hunger.
PositiveTip: For good health, we must eat with our needs in mind, not just for pleasure.