This Blog Post, "10 Types of Odd Friendship You're Probably Part Of", is an excellent primer for paying attention to one's relationships. It's an important skill to have if one wishes to live a secure and fulfilling life.
There is certainly more that should be said concerning the nature of one's long-lasting friendships and the circumstances in which one forms said friendships. Such a discussion would necessarily concern itself with the nature of family/tribal life, education vs. school, and other anarchist talking points, but this article itself rightly chooses to give a very high-altitude normative statement.
For instance the "I stole you from your former friend" friendship. One in which you were introduced by a mutual friend and promptly forgot your mutual friend in favor of the new one, which could create tension in your new relationship, especially if you frequently find yourselves in a group with the original friend.
Or another one would be the "weird breakup friend" friendship. One in which there is an unresolved sexual/romantic issue in a small group. For instance, a semi-successful attempt at "we can still be friends", or "those two were going out, but they broke up and both still want to be in the group, but without the other one", or "we both slept with him/her and have some of his/her dirty laundry and need to keep it secret from the group. Such Drama, Much Immature, Wow.