First of all, I just want to say that this fan theory is not yet released publicly until now. Well, I'm the theory creator. After watching "The Dark Knight" 5-7 more times and reading some articles about how Heath's Joker was formed, I've came up with a theory. Some articles are really giving the saying of Joker "Wanna know how I got these scars?" a hint of his past. So I'll be elaborating the story and adding my few details to really have a story with sense.
That was already a hint at his past. He had an abusive father. But why? I'll tell the reason why later. But first, let's give him a name. I came up with something pretty good. James O'Kerr. Yes, I know it's pretty corny but it'll, also, have sense later. The father of James was always angry about him being too serious when he was around. Why? Because James was gay. James tried to hide that from his father. He decided to be his true self when his father was not around. So when his father is around him, he'll act like a true man. One day, James' father drank too much. Then the Joker's story applies. But before that, James' father realized that his son is gay. That is where the sentence "Why so serious?" came intact.
After some years of torture from his dad, he decided to leave his house and to live alone. One day, he stumbled with his dad. His dad still questions his gender. James' then said that he'll go to the army to prove to his father that he isn't gay anymore. And he did. He had a dog tag that he always carried with him that has the name J.O'Kerr in it. After a while, he met a woman in the army. Named Barbara. Yes, that Barbara Batgirl. And they fell in love. Blah-blah-blah. But the army knew about his past, that he was gay. The army then decided to fire him. But Barbara still was communicating with James after that. After a year, they got married. Yet, the army didn't like that.
Barbara was fired, also. And she didn't just get fired off the army, she was punished by her fellow women army. That's when Joker's story again comes intact "One day they carve her face. And we don't have money for surgeries. I just wanna see her smile again." Barbara was really depressed that she didn't have a job, has an ugly face and a husband who was gay and also doesn't have a job. That's why she couldn't smile. But the army offered her another chance. But if she breaks up with James. So the day that James sticks a razor in his mouth and does the chewing thing. Then she leaves. James doesn't know that she just accepted the army's offer. "Now I see the funny side. Now I'm always smiling!" That is the realization of everything he has will leave him (Well, technically he left his parents. But still he lost something). That same day, he had an accident. Batman bumped him. He, actually, recognized everything. From Batman to the Batpod. He had an amnesia. But there are some details of his life that he remembers. But he remembers them in different dimensions. He remembers his father but not his wife. He remembers his wife but not his dad. H remembers the the army but not his wife. He remembers Batman bumping him and not helping him out. He remembers something in a different dimension, and another in another dimension.
In the accident, his fingers were scratched. his teeth were lost or rather some of it. Remember that scene where Gordon came back from the dead, he said "No matches on prints, DNA, dental." That's how he lost all of that, and remembered only a victim of Batman. On his awake, he saw his dog tag. With "J.O'Kerr" in it. He laughed. Saying it was wrongly spelled. And that was the creation of Joker. So to make the long story short:
James O'Kerr - gay - abusive father - army - Barbara Gordon - fired - marriage - carved face of Barbara - new offer for Barbara - left - Batman accident - Joker.
So basically that was my edition of Joker's past.