As Daniel Cervone states in this program, the reason is that each individual is different and a good theory must take into account these individual differences and apply them universally.
In studying individual differences in behavior, psychologists like Gordon Allport recognized certain consistencies that were believed to be biologically driven. These consistencies, or characteristic patterns of behavior, thoughts, and feelings, account for the traits an individual possesses. The combination and interaction of various traits is what makes up personality. In the 1930's one of Allport's students compiled a list of thousands of stable characteristics, which helped to develop a theory that reformatted these traits in a more simplistic style.
British psychologist Hans Eysenck suggested that personality was based on just two dimensions: extroversion and neuroticism. Each individual trait, he believed, could be charted somewhere along each of these dimensions. Eysenck's model, which was criticized for focusing on too few traits, was soon replaced by slightly more complex theory called the Big Five model of personality.
The Big Five model is comprised of five personality dimensions: conscientiousness, agreeableness, neuroticism, openness to experience, and extroversion, which form the acronym "CANOE." As with Eysenck's model, these factors are dimensions, not types, so people fall somewhere on a continuum. Studies have shown the factors proposed by the Big Five model to be consistent cross-culturally.
So, where do these traits come from? In the final part of the program, Cervone explains that the Big Five model makes the presumption that traits are rooted in our biology. Twin studies have been especially useful in showing how much our genes influence who we are. The study of personality requires taking into account the whole picture—our biology, our environment, and how the two interact.