I find the ties that bind to be a very interesting topic. What causes people to come together, to enjoy one another's company, and to be friends. Often it is a commonality that brings two people or more together. What I find interesting is that it is generally the lack of a certain attribute or belief within a microcosm that will cause people who poses it to become friends. For example, I am left handed, no one would think of being right handed as a basis for friendship, because you would have to be friends with nearly everyone. Being left handed on the other hand makes you unique and when you find another person with that uniqueness it binds you together. Several of my close friendships have been with other south paws, yours may have been to but it may not have been defining for you the way it was for me (unless of course you also are left handed, in which case we are probably very good friends, or would be).
The more a group of people are the same the more minute the details that make friends, the more a group is different the more broad those attributes can be. For example in most places two people being mormons is a reason for them to bind together as friends, they immediately know that certain things about them will be the same and different from those around them. In Utah this is not the case, nobody is friends simply because they are both members of the Church, you would have to be friends with everyone you met, and that would be overwhelming. This was very prevalent on my mission with Africans, whenever my friend Elder Fayanni met another African there was an immediate bond just on the principle that they were from the same continent. They may not even speak the same language natively, in Africa they may have wanted to kill each other, but in Philadelphia that continental heritage was enough to forge an instant friendship.
This is found everywhere you go even if you don't always think about it. At the root of most of our closest relationships is a commonality that at least brought us together initially and may still be a binding factor in the friendship.
I think a major reason for this is that sharing a common set of experience or knowledge cuts the way more quickly from a very surface form of communication we use when we first meet someone and allows you to dive deeper into things that are more meaningful and personal. You know there will be things you wont have to explain to this person and you know that they will understand simple references, or certain feelings. This is a wonderful thing although sometimes it is very healthy to interact with people who have little in common with us so that we might have to explain things and thereby better understand ourselves.