An extract of an interesting article, though I don't necessarily agree with it all:

Alchemy in Relationships

by Hajo Banzhaf & Brigitte Theler

"Self-sufficiency and symbiosis embody the extreme forms of two basic forces that, lived in a healthy balance, control every living relationship -- dissolving and binding. In every relationship these two forces are in balance with each other. Whether this balance of power comes about in a relaxed and harmonious way, or whether it is achieved after repeated, hot wars petrify into a state of cold war, or clothes itself in apathy and indifference, is all the same. Both of these powers are balanced in every situation.

In this process, it may look like one of these two forces has temporarily been repressed and the other has won the upper hand. But it is quite certain that the seemingly inferior power is only repressed into the unconscious and will re-form itself there in order to appear again sooner or later and win the upper hand. The roles belonging to this interplay of forces are often already given to the participants at the first moment of meeting each other, long before even one of them suspects that a relationship will develop from this encounter. From the very beginning, one of them takes the role of "binder" while the other takes over that of the "dissolver." And this is how it usually stays thereafter. Only in rare cases does an exchange of roles occur during the further course of the relationship ..."