Teal Swan made an excellent video on the topic of Relationship Addiction.  She brings in science of how mice, when given intermittent reinforcement in the form of pellets, they become crazy obsessed, camping out at the pellet dispenser, pushing the button, trying to get a pellet.  When a pellet is dished out intermittently, the mouse amps up the obsessive behavior.  But if the pellets were given consistently in the beginning and then suddenly stop, the mouse is likely to move on to something else, likely deducting that the source had run dry.

In narcissistic relationships there is normally a ton of reinforcement in the beginning and then the reinforcement slowly becomes intermittent, creating an addiction in the person who is holding out for the attention, affection and love from the narcissist.  I would highly recommend watching this video for anyone who suffers love addiction or addiction to a narcissist.