Friendship seems to be the less stable and more likely to change than romantic or family relationships because friendship has a unique relationship based on person to person, and how others connect with one another. This is different than a romantic relationship because we are not met with intimacy. Friendship is not as stable as family because family has more of a united structure and the family relationship is admired and dominant in our society. Friendship, on the other hand, is sometimes seen as derogatory or negative in our society.
I agree, that it appears that we are more prone to cutting off our friends than our family or romantic partners. I think this is because we do not feel as though we need friends to be successful or to encourage us along our journey. Family and friends can provide the support and love that we can get from friendships.