So let us examine the relatonship dynamic that has taken place. This is due to the unconscious attractor force that brought them together. A and B where independent enough at this point to be focusing on a particular set of values or lifestyle choice that caused this attraction to take place. A and B spend time together and the attractive force between them strengthens. This bond grows to a level greater than anything else in their lives and eventually in most cases this is the point that the balance is upset.
Balance is about valuing ones own self more than anyone or anything.
Most relationships reach a point of dis-equilibrium where one of the parties becomes more focused on the other than on themselves. When this happens the downward spiral begins. Soon after this the dependancy that is brought into the relationship will soon wreck it. This is because we are not valuing ourself and in any relationship we simply get back from the other party a reflection of how we view our inner self. Hence, when this value is no longer there sooner or later the other person will feel the same way.