"Because psychopaths are great manipulators and convincing liars, as we’ve seen, many of their victims don’t heed the warning signals. During the early phases of a romantic relationship, people in general tend to be too blinded by the euphoria of falling in love to focus on noticing red flags. Also, during this period, the psychopaths themselves are on their best behavior. Yet, generally speaking, they get bored too easily to be able to maintain their mask of sanity consistently for very long. The honeymoon phase of the relationship usually lasts until the psychopath intuitively senses that he’s got you on the hook or until he’s gotten bored by the relationship and moved on to other targets. He shows his true colors when he’s got no incentive left to pretend anymore. As Babiak and Hare note, “Once psychopaths have drained all the value from a victim—that is, when the victim is no longer useful—they abandon the victim and move on to someone else.” (Snakes in Suits, 53)
This raises the question of why a psychopath idealizes his targets in the first place. Why do psychopaths invest so much effort, time and energy into giving the illusion of intimacy and meaning in a relationship, given that they never really bond with other human beings in the first place? One obvious response would be that they do it for the sport of it. They enjoy both the chase and the kill; the seduction and the betrayal. They relish creating the illusion that they’re something they’re not. They also enjoy observing how they dupe others into believing this fiction. Moreover, whenever a psychopath expresses admiration, flattery or enthusiasm for someone, it’s always because he wants something from that person. I think, however, that this explanation is somewhat reductive. Many psychopaths experience powerful obsessions that resemble intense passions. Besides, this explanation doesn’t distinguish conmen, who fake their credentials and interest in a person, from psychopaths “in love,” who are pursuing their targets for what initially seems even to them as “romantic” reasons....."
See more about The Psychopath's relationship cycle