Maybe wasted effort, but I think there is so much going on that isn't being pointed out.
Joker being clinically psychotic, for one. Is someone in his shoes capable of feeling that deep a 'good' emotion? I mean, he is essentially living in an unending psychosis (Quote from his profile on DC Wiki: "He is constantly adapting his personality and his psychosis to respond to the world around him, sometimes a harmless trickster and at others a brutal mass-murderer."), and that's ignoring his sadism and narcissism sprinkled on top of it.
That being said. Though it is possible for Harley to be blindly in love with him, I highly doubt that. This isn't some high school cheerleader that is in love with being in love. This is a mastermind and psychiatrist that worked with Joker during his time in Arkham Asylum. It can be argued that she is deluding herself, but that kind of delusion won't stand up to multiple attempts on her life, not with her intimate knowledge of the inner workings of the mind.
In other words, he can't be in love in the sense we understand it. That's not possible, not in his situation. However, it is possible that he loves her in his own, sick, twisted way and that she understands enough of his inner workings to appreciate that about him. Hence their twisted off-on relationship.
But maybe that's just me fangirling on Harley ^_^