This is the most puzzling and noteworthy question to be answered in psychology, theology or philosophy; however there is no satisfactory answer till present. Few reasons are there, due to which we are unable to answer this question. One reason is the individual differences in human beings. World have population in billions but there are no two for individuals who are exactly the same. Even the identical twins have different personalities. The answer is difficult because the human come in different shapes, belong to different culture and race and human behavior is complex. Human differences can be found in term physical, psychological, sociological, genetics etc. So human is a complex type of creature where individual differences make different personality types. What do you think, type in comments.