The problem with codependency is that it creates a detrimental behavioral interaction in a dysfunctional relationship. In other words codependency allows two people to function improperly with one another. The person that is codependent can be manipulated and even controlled, while the other is the controller. Codependency issues require that a person seek help in order to regain their life.
For example a situation where the controlling party is a drug addict can make it difficult for the codependent one to cope well. In this situation of codependency the controlled person may not be able to help themselves. They would need someone else to actually step in and remove the controlling factor. Codependency requires a skilled psychologist to help resolve the issue by working through the behavioral issues the person has.