I was discussing this w/ someone as to when the Joker actually started caring for (read: have an annoyed tolerance for in the eyes of most people) Harley. It must've really pissed him off b/c he couldn't control it. In Mad Love, I felt like it was when she broke him out of the Asylum. In Batman: Harley Quinn, I thought it was when she showed up again, dressed up in her HQ outfit.