Mechanics of Attachment
One way that people become more attached to each other is that once you have shared some story or information about your life with someone (a friend, lover, etc.) whenever there is an update to that story or a change in the status of what you have shared, you want to share the update with the person who already knows the back story. It doesn’t really work to retell the whole story to someone else because it they don’t have the correct setup time to be truely surprised with you about the change in the status of the story. For example, if you have been telling someone about how you are taking a group exercise class and how hard it is, you want to tell them that you passed your final by 1 point! It doesn’t really work to tell someone else that you have passed your course by one point because they don’t have the full emotional impact of the new information.