Over the last decades, various researches have been done to evaluate how and why people are different. People are eager to know why they are different. This is an important subject in daily life as these differences cause different attitude towards life, relationships and jobs. Hence, researchers are psychologist have done several tests to determine some criteria to study people based on them. However, some of these factors are more valid and reliable. This paper proposes a study of personality and intelligence as two main universal factors that cause differences in people. Some of the personality tests are discussed over time to specify their flaws and main factors in intelligence are studied alongside the brief study of the IQ test. Furthermore, some of the most effective and well-known aspects of self in relationships are studied to help people first understand themselves better and then, to have more intimate and efficient relationships with others including the loved ones, friends, co-workers. Therefore, the main purpose of this paper is to discuss how and why people are different from one another.
