Remember when young Bruce Wayne fell down that well? Reddit user killjoy95 suggests that maybe young Bruce experienced PTSD from such a traumatizing fall. Given that he's lived such a happy, privileged life, falling down a well as a young boy isn't exactly easy to get over.
The theory goes further, according to Reddit user slugboi. In addition to the PTSD of falling down a well, Bruce lost his parents at a very young age. The trauma from that could've sent him over the edge.
Young Bruce was growing more and more unstable, so he was sent to a children's psych ward. This was a relief to the board at Wayne Enterprise, since there's no way he was mentally capable of taking over the company.
Dr. Joe Car is the doctor that believes in Bruce. Reddit user VikingSlayer suggests that he tries to reach Bruce through jokes and friendly demeanor, but Bruce's mind warped him into a villainous clown.
Dr. Joe Car's assistant is none other than Dr. Harleen Quinzel. This checks out because according to the comics/animated series, Harley Quinn's real name actuallywasDr. Harleen Quinzel. She fell for the Joker, so maybe Bruce's fragile mind twisted it into a warped relationship.
Many hospitals make the surroundings as green as possible because green is a color associated with stress relief. So, to keep Arkham a happy place for the patients, the hospital hired a groundskeeper to tend to the plants around the building, who Bruce names Poison Ivy.
At an insane asylum, you're bound to have a lot of turnover with the janitorial staff. Since they always wear the same uniform, Bruce Wayne thinks it's the same person, and that this person has the ability to change his face.