Human existence is regulated by a very precise reward system where most actions are motivated by an urge for a specific biochemical kick!
This means that most of human life is an ongoing pursue of these biochemical fixes.
This system was designed as a way to direct instincts and get the animal to behave in certain specific ways.
As human beings this system still regulates most of our animal or instinctual nature.
It regulates for instance our instinctual drives in procreation, survival, social power, ambition, emotions and so much more!