There are many different ways to classify people into "groups." You can, for instance, separate people by gender, by religion, by race, or by a number of other different subsets or preferences, such as "does the person like vegetables?" Once people are separated into different groups, they often tend to stay within that group for reasons such as lack of conflict and having things in common. If you love a TV show, and you have a group of friends who also love that TV show, what reason do you have to try to separate? Similarly, many people choose to remain in cohorts of their biology. Many people choose to befriend people of similar races, genders, hair colors, ages, etc. This is associative, because people assume that people who look the same will, in some facet, also behave the same. By remaining with people who are like you, you will limit potential conflicts of interest. The issue there is that you will never grow. Meeting and learning about other groups of people...