Melody Beattie wrote a great book in 1987 called Codependent No More. There was a movement in the late 80's called Co-dependency annonoymous. It was a 12 step program that helped people realize that they may also be powerless over relationships. Those meetings are few and far between today.
Co-dependency means several things. When you constantly take someone else's inventory or become obsessed with how a friend, family member or spouse lives their life you may suffer from it. In the case of obsessing over an alcoholic or someone who appears irresponsible it is easy to feel like you are "one up" on them. The problem is it is not your job to police the universe or try to control another's behavior.
Under the guise of caring for others we justify this behavior and if we're not careful it can become a very self destructive way of being. Teenagers are a natural at this because they get caught up in everyone's drama. It may be part of the developmental process. However, there is a way to become aware of it, set appropriate boundaries, and spend more time on caring for your self. Another clue of co-dependency is being attracted to people who are really not available to you. You then spend all your time trying to change them. Relationships are just easier if you find someone that doesn't need fixing.