So I was reading Batman and Robin adventures and I find this.
In this chapter/issue the joker doesn’t find anything funny anymore and Harley is doing a lot of stuff to make him laugh again, and then when this man hits Harley, the Joker gets mad at him for hitting her.
This shows that he is the only one that can do that to her, because it’s his way to express love, because he doesn’t know any better.
Like I always say: he may not be able to love her like regular people do, but he does love her as much as a psychopath like him can.