This is the paradigm cycle that people exist in regarding narcissistic relationships. It is very often an addiction. But while many believe they are addicted to the other person, the truth is that both people are addicted to the drama, chaos and intensity of the relationship - the emotional payoff. They are each others' trigger, creating a desire to use (create drama/chaos) when they are around each other.
Listen to Breaking The Addiction to a Narcissist for more information.