"The worst part of having a mental illness is people expect you to behave as if you don't."
This is something that we see written in Arthur's joke journal. It is a window into his mental state and the fact that he feels so alienated from mainstream society. It also clues us in to the fact that Arthur feels inhibited by other people's expectations that he keep his mental illness under wraps. Not only must he cope with the fact that he has a mental illness, but he feels as if he must keep it hidden, which makes it all the more excruciating.
"My mother always tells me to smile and put on a happy face. She told me I had a purpose: to bring laughter and joy to the world."
This is Arthur's mission statement. Although incapable of doing so, Arthur wants to leave the world a better place. He wants more people to laugh in the world and wants the world to be a more joyous place. In the film, he says this in his fantasy about speaking to Murray Franklin and being celebrated as a hero.
"They don't give a shit about people like you, Arthur. And they don't give a shit about people like me either."
When Arthur visits his social worker in the middle of the film, she informs him that they are disbanding her office. Here, she tells Arthur that the system does not care about people like him or her, and that they are victims of neglect. This starts to get Arthur thinking about the way that he has been abandoned and marginalized by the authorities, which sets him on his track towards becoming Joker.
"I had a bad day."
After he finds out that his mother was committed to Arkham State Hospital after suffering from delusions and allowing Arthur to get abused by her ex-boyfriends, Arthur goes to Sophie's apartment. When she finds him there (and the audience realizes that he let himself into her apartment without her permission), he says this, looking for some sympathy in a cruel world.
"What do you get...When you cross...A mentally ill loner with a society that abandons him and treats him like trash?! You get what you fucking deserve!"
Arthur screams this off-color joke at Murray Franklin when he appears on his show, just before shooting him in the head. It is a moment when Arthur is perhaps the most articulate about what has happened to him, even if it is in the form of a disturbing "joke." Here, he suggests that he has become a killer because society has treated him "like trash." He calls attention to his own victimhood.
"You wouldn't get it."
This is the last line in the film, which Arthur addresses to a social worker at Arkham State Hospital. When she asks what he is laughing about, he smiles and tells her she would not get it. In this moment, we see that Arthur has descended completely into alienated villainy, unable to connect with or even explain his logic and thoughts to others.
"You know what’s funny? You know what really makes me laugh? I used to think that my life was a tragedy...but now I realize...it's a fucking comedy."
Arthur says this right before he smothers his mother to death with a pillow at the hospital. He has become completely unhinged, pushed past his own limits and completely deranged, believing that the most tragic situations are actually comedic.
"You're so funny, Arthur."
Sophie says this to Arthur, in what we later learn is a fantasy, after confronting him about following her that day. In Arthur's fantasy of her, Sophie thinks he is funny and that it is charming that he has basically become her stalker.
"And finally, in a world where everyone thinks they could do my job, we got this video tape from Pogo's Comedy Club right here in Gotham. Here's a guy who thinks if you just keep laughing, it'll somehow make you funny. Check out this joker."
This is how Murray introduces a clip of Arthur bombing on a comedy stage. He ridicules Arthur and draws attention to the fact that Arthur is not in on the joke. It's a moment of cruelty from someone who Arthur really looks up to and admires.
"He's an extraordinary man, Happy. A very powerful man. We were in love. He said it was best that we not be together because of appearances. And I couldn't tell anyone because, well, I signed some papers. And besides, you can imagine what people would say about Thomas and me, what they'd say about you."
Penny, Arthur's delusional mother, maintains that she and Thomas Wayne once had an affair. Here she shows Arthur that she still holds out hope that Thomas will take care of them, and sympathizes with the billionaire.
