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 is a valuable piece of education that gives you incredible human qualities such as empathy and understanding. If you are of one religion, you will likely be attracted to others who also practice that religion. But if you only talk to people in your religion, you will never learn about other religions and other beliefs.
There is a difference between trying to convert people of other groups, and interacting with people of other groups. If you are a female and you only communicate with males in order to convince them that females are the best, you have not developed a meaningful relationship outside of your group. However, if you try to learn from your male friend about his experiences in life and how they differ from yours as a female, you will become a more wholesome individual.
Many people are afraid of being "wrong," or of having a difference of opinion with another person. That is why groups do not often attempt to extend their hands to people of other groups. However, if you make the effort to become a more educated person by learning about the experiences of other people with other opinions, you will grow as a person considerably more than you would if you limited yourself to being surrounded by people who are exactly like you.