The Abusive Dynamic
It will come as no surprise to learn that The Joker is an abusive boyfriend. He is clearly a homicidal maniac with little regard for the lives or well-being of others. He is also deeply manipulative, and generally stops at nothing to get what he wants. In addition, he is locked into this obsessive situation with Batman, which can’t be helpful in terms of maintaining a relationship with someone else. It is, in many ways, his narcissism that defines his relationship with Harley Quinn.
If his personal agenda is one of self-service, then Harley serves an important purpose for him. It is easy for him to take advantage of her devotion to him – seeming to manipulate her into helping him escape when necessary, or helping him execute various nefarious plots as he sees fit. Once she has served her purpose, he rejects her, however – even going so far as to kick her into a vat of chemicals at one point. She is useful to him though in that she will always undertake actions that he is not willing to commit himself – usually out of self-preservation. In his addled mind, this is the equivalent of finding somebody attractive, or wishing to have them in your life.
For Harley, the relationship is like an addiction. In exploring his unhinged mind, Harley found something she did not realize she had been seeking – a different way of life. The Joker is a powerful and dangerous man precisely because he is seductive, charming and highly intelligent. During their therapy sessions, he figured out Dr. Harleen Quinzel very quickly, and assessed what she needed to hear in order for him to achieve the result he desired – having an ally to help him. While he may not have anticipated the extreme psychotic break she would eventually experience, this would also be to his advantage, ultimately.
In order for him to be successful in his initial manipulation of Harley, he needed to pick up on some weaknesses that could be exploited, and some buttons that could be pushed – and this is, perhaps, the most important aspect of this affair. There is a reason that Harley Quinn is driven insane – and The Joker is only a small part of that explanation. This is what makes Harley such a fascinating character in her own right, and what makes her relationship with The Joker so compelling.