The first part of what I wrote in my Cursed Notebook™ instead of sleeping/doing my schoolwork a month ago. Basically what I did is I watched the movie one time through, then started it over from the beginning and stopped after every scene to recap and infer what Arthur’s inner thoughts/motivations are. I went through an entire pen writing all this down and I was also very sleep-deprived so prepare yourselves. (I’ve also seen this movie 12 times now so I’d like to think I know what I’m talking about, but if you have any concerns, just message me!)
Before the Movie (+ Diagnosis):
Arthur Fleck is completely alone in this world, with no one to lean on except for his mother. And he is forced to take care of her due to her old age and fragility, all the while facing near-total isolation and societal backlash for just trying to do his job or his neurological condition/mental illness.
Given the abuse he faced as a child, at the hands of his mother's boyfriend, and the head trauma explicitly mentioned later in the film, that could be the origin of his Pseudobulbar affect (pathological laughter). Pseudobulbar affect (PBA) "is a condition that's characterized by episodes of sudden uncontrollable and inappropriate laughing or crying. Pseudobulbar affect typically occurs in people with certain neurological conditions or injuries, which might affect the way the brain controls emotion," and has "traumatic brain injury" as a common cause. The way I see it, the "severe trauma to his head" he experienced as a kid is the direct cause of his pathological laughter, meaning that he has spent the vast majority of his life dealing with this condition and it is mainly his own mother's fault. This makes the fact that his "mother" is the only person he has left even more frustrating and tragic. Pseudobulbar Affect is also known to cause/amplify anxiety, depression, and social isolation, meaning his obvious depressive symptoms may stem from the rejection he feels due to his laughing condition.
Also, although I know that even the writers behind the film say that diagnosing Arthur is pretty much useless, I still feel it is important to mention some possible diagnoses. Due to his severe depression throughout the majority of the movie, one could assume he simply has Clinical Depression. However, when paired with his transformation into the Joker at the end of the film and his obviously manic state, this leads one to believe that Bipolar Disorder could be the culprit. But then there is also his hallucinations and delusions, which suggests the presence of some type of schizophrenia. Psychosis, or loss of touch with reality (exhibited by the aforementioned hallucinations and delusions), is a symptom commonly linked with schizophrenia and similar disorders, and is typically caused by trauma and extreme stress. The psychotic episodes we see in the film, from his relationship with Sophie to the audience reactions we hear when he's practicing for the Murray Franklin Show, all center around either erotomanic delusions or grandiose delusions. Erotomanic delusions are when the disordered person believes someone is in love with them with no real evidence (Sophie), and grandiose delusions are when the person believes that they have much greater worth and power than they do, and that they may be famous with a bunch of adoring fans (the Joker persona).
With the presence of psychotic episodes, this narrows the amount of possible diagnoses. It is unlikely that Psychotic Disorder or Paranoid/Hebephrenic Schizophrenia are responsible due to their explicit tendency to completely disrupt the sufferer's life and negatively affect communication/speech patterns (which isn't noticeable in Arthur's case). Delusional Disorder seems likely, but he meets too many of the criteria of another disorder for it to be his sole diagnosis. This disorder is Schizoaffective Disorder, and is "a chronic mental health condition that involves symptoms of both schizophrenia and a mood disorder like major depressive disorder or bipolar disorder.” This diagnosis seems much more apt for Arthur, due to the heavy emphasis on his depressive and manic states and the delusional/hallucinatory symptoms. Schizoaffective Disorder is also likely caused by extreme stress (which Arthur faces a lot of) or structural brain issues (head trauma can cause structures in the brain to be damaged).
Sorry for the long tangent, but I felt it was necessary to set the scene for how Arthur processes the events that happen to him throughout the film.
His mental illness and condition make his life incredibly difficult and emotionally draining. Even his job, which is something he seems to really enjoy, only causes more problems for him, and pushes more people away. And even though he loves his job, he dreams of being comedian rather than just a party clown, probably hoping for more respect while still doing the thing he loves: making people laugh. His sense of humor is rather off-beat, focusing on self-deprecation, dry humor, and sometimes revolving around morbid subject matter, so he instead changes it to better fit what style of humor is generally accepted by those around him. He really tries to do what he wants to do in life, but everyone around him seems to just want him to sit and stay quiet. No matter what he does the people around him are never satisfied. He's doing too much and too little at the same time. He's trying to gain recognition, and people stop that from happening, then he just tries to lay low and let life happen and people look on him with disdain, as he's just another poor person who "isn't trying hard enough" to escape his current life of near-poverty. His job pays the bills, and is the sole source of income as Penny is obviously to ill to work.
No one wants to stop and give him a chance. He feels as though the world is getting more and more incompatible with every aspect of who he is. The city he's lived in his whole life is getting buried in trash from the garbage strike. His mother is getting sicker. The therapy he’s getting is sub-par and the social programs he relies on are gradually being defunded, another way people are shoving him aside. His dreams are as far away as they've ever been, or maybe even further away, and it feels like he is going nowhere but down. He's grown disillusioned with the idea of socializing in order to solely get to know someone because no one ever does the same for him. People ignore his existence, and his mother, the only person he has, is only there because she's too old and sick to have a choice. He's trying so desperately to be happy but it's painful.
