Warning! Spoilers for Batgirl #47 below!
There's no question that the Joker has an unhealthy obsession with Batman. The supervillain's actions are always in direct conflict with the Caped Crusader. However, when the Joker tries to get Batgirl to give him vital information to continue his takeover of Gotham, Barbara Gordon fights back with words by telling him he's in love with the hero.
The Joker is the rare villain who doesn't want to kill his main heroic adversary but instead relies on Batman's existence to keep going. As Heath Ledger's Joker famously said in Batman: The Dark Knight, “I don’t want to kill you! What would I do without you? You... you... complete... me.” Without Batman, the Joker would feel completely lost and it's debatable whether he would even both continuing on as a villain - a concept that was explored in a recent short story. So, when the Joker tries to play mind games with Batgirl, Barbara fires back by exposing his love for Batman.
Batgirl #47 by Cecil Castellucci, Robbi Rodriguez, Jordie Bellaire, and Andworld Design, the Joker breaks into Barbara Gordon's apartment and reveals he knows her true identity. This obviously greatly upsets Barbara, as she was paralyzed by the Joker in The Killing Joke and still struggles with PTSD related to the incident. He disables the implant in Barbara's spine that allows her to walk and sits down next to her on the couch. He tries to intimidate Barbara in order to get codes for the Batcave now that he's overtaken Batman's resources. He calls her a "pale imitation of Robin." But Batgirl fights back by calling the Joker "desperate for his attention," and that he wants to be the "Batman's sidekick." She says "you love Batman... you can't live without him! Who's pathetic now?" while laughing in his face.
Barbara clearly knows how to get under the Joker's skin. Calling out his love for Batman and laughing at him is about as confrontational as she can be to him. Her laughter forces the Joker to get quite upset. It must feel nice for her to toy with the Joker's emotions considering all the pain he's caused in her life. She understands his feelings for Batman and used it against him.
The scene shows just how far Barbara has come as a character. While she was understandably panicked by the Joker showing up, she drove him mad by exposing the cold hard truth to him: He loves Batman and can't live without him. The Joker might not accept that as fact, but his actions in the 'Joker War' surely paints the picture that his unhealthy obsession runs deeper than we can ever imagine.