I think the Joker is a far stronger and more fascinating character if he does deep down love Harley. Did it start out that way? No. But as Mark Hamill once said “Expressing emotion in any way that’s real and meaningful is alien to the Joker, but he’s learning those parts of himself, however unconsciously, through Harley. On a physical level they’re dynamite together. A lot of relationships are defined by that. Two people may be really horrible for each other, but physically they push each other’s buttons so clearly they can’t get enough of that person.” Arleen Sorkin once said that “Everyone else sees the Joker laugh, only Harley has ever seen him cry. It’s the only reason she stays with him.”
Weather or not the Joker loves Harley really depends on whose writing him. I’m HUGE joker fan and I must say that I find the character to usually be written rather 2d. An irredeemable psychopath who was a child hood social path, asexual with an obsession with Batman who uses people like Harley for a means to an end. This is an important interpretation of the Joker which has been mastered through Serious House on Serious Earth, death of the family, Jack Nicholsan’s Joker, and despite Mark Hamils comment the dcau Joker never really seemed to care about Harley or anyone except himself and defeating Batman.