There is no story without characters. There are no characters without motivations to guide their actions. In order to create an organic, reacting, human character you must understand why they do what they do.
Sigmund Freud, father of modern Psychology, inventor of "Yo Mama" jokes
Often your character’s actions will be driven by impulse: lust, anger, fear, or any one of the 61 other possible human emotions. However, this article explores large-scale motivations, the consistent traits that define them as a person.