There's no doubt that Joker initially wooed Harley just to break out of Arkham, but there is a scene that I really love later on where Joker asks Harley to "live" for him and she willingly jumps into the vat of chemicals that first created him. We then see the Joker thinking of leaving her to die, but he ultimately jumps in after Harley and resuscitates her.
The Joker is basically pure id that only operates in the extremes and, to many Leto fangirls' disappointment, doesn't really go for shades of gray. Once he decided to save Harley, he essentially decided to love her with all of his sick mind. That's why he spends the entire movie trying to rescue her. It isn't part of some kind of master plan. He just wants to be back with the woman he loves.
That's what makes the "love" between these two nutjobs so terrifying. Their goal is simply to be together. Once they have that, NOTHING else matters, and they allow themselves to be crazy and psychotic together, like during that fantastic "You're ruining date night!" scene. Joker and Harley are so in tune in the malfunctioning brain spittoon that when he tells her she suddenly belongs to some gangster (who was making him tons of money btw), she knows instantly what he means. He wanted to mess with the poor schmuck's head before adding a sunroof to it, all because the guy called Harley Joker's "bitch."
The kind of love that we see in Suicide Squad is made out of pure passion that surprisingly never turns into abuse towards the other person. It's only lethal to those stupid enough to stand in its way. Does any other movie do something this fascinating? The answer: Yes, Natural Born Killers, but that movie's like 23 years old, and it's high time we got a new version of that story.