What ought to motivate human behavior?

Human nature is the driving force. Humans are living creatures and all living creatures have the same innate driving forces; to support life (eat, drink, protection from the elements), to defend life (in humans the response to blink or duck are forms of that), and to reproduce (is it any wonder that sex sells, it's a force that will not be denied). Unless a persons nature has been distorted by circumstances or corrupting influences, most of everything that humans do can be traced back to these forces.