As human beings, we need variety in social interactions. Not even variety in the amount people, but variety in the types and facets of people. If we don't have that variety, it can create a dependance on that person, and create a sense of frustration because they can't do everything.
in an inter-dependant relationship, both people can be independant, healthy, human beings, in a codependent relationship, you start subsuming other, and it can develop into a lack of identity, rage, and frustration.