He hits her cause
she likes being hit. (Poison Ivy and Batman tell her that as well)
See, Harley has a psychological and emotional dependancy to Joker that is directly tied to abuse because she bonded with him and fell in love with him durring their therapy sessions.
So while he would tell her stories of himself being abused throughout his childhood, she was creaming her panties falling in love with him.
Likewise, he was becoming attached to her while telling her more and more horrible scenereos. So now, the two of them have equated abuse with love, and because they are both insane its an extension of their relationship.
That strip someone posted of him wanting her to cut her face is like him asking her to marry him basically.
The variant cover to Future's End explains it perfectly. Two covers for the same issue that we see as being very opposite.. but Joker / Harley would have a hard time explaining the difference between the two.
...for real??
These sensitive ass people. 
