A huge shout out to Reddit users, you guys are awesome and make my fan theories worth pondering, even if I make mistakes! <3 you guys.
So I have been re-watching Batman and suddenly a theory popped in to my head as to why the Joker specifically became the character that he is.
The Joker is deadly fearful of Clowns and other performers.
In the same way that Batman turned his fear of bats into his greatest tool, the Joker turned his fear of clowns and other freak shows into the most powerful asset of evil.
Why is the Joker afraid of Clowns and other performers though?
I think the many stories of the Joker's father gives a look into the psychology of the Joker; at one point he was a normal child up until he attempted to make his father laugh at some point, at which point his father went apeshit. In both the comic, the animated series and the new trilogy, we see the Joker give various stories of how he was abused - the common root is that he tried to please his father through laughter, to which he was severely punished, and there-on from was twisted, then when the incident of chemical waste occurred, all those mental instabilities burst open, with his psychopathic nature embodied by the performer, being released.
A petty mod-hound was turned into Gotham's greatest monster.
The Joker used the Batman philosophy - turn your greatest fear against others and make them afraid of it too.