My personal theory on his background is something I would like to see, but I know that it is highly unlikely, mainly because it strays from the acid/chemical tradition.
I never put a name to it, but I always imagined Joker as a Catholic white male, who was bullied at school and at home by his aggressively religious parent/s. He would always be told, by his parents, to 'turn the other cheek' while he was getting bullied, and to laugh it off. So throughout his turbulent life, he would be bullied, and he would laugh about it. This would earn him names like 'Clown' or 'Joker'. Anything at the school that could be pinned on him by his tormentors would receive the same treatment for him.
He was very fastidious in his studying and did fairly well, getting a job in a large conglomerate. Without the bullying, he would have a semblance of life and friends, but he felt empty, just like he did at school. His life would be fine until the day of the accident. While driving home, a little girl darts across the road to get a ball. Despite his best efforts, he kills her. While he races out of the car to check on her, he does the one thing he did whenever something bad happened to him. He started to laugh. The other people who ran to the scene would see this man, laughing at a dead girl. He would try to explain, but because he hadn't laughed for this reason for so long, he can't stop. The crowd are less than impressed and assault him. Which kept his laughter going, until he was knocked out.
Waking up weeks later from the induced coma he was put into to give him a chance, he remembers the incident, and he doesn't laugh. For the first time he has to fully accept the consequences of his actions. He caused the death on an innocent little girl. He could go into confessional, but a soft chuckle rises in him which he stops. No, he can't do that as much as he would like to. He can't kill himself as suicide is also a grave sin against his religion. Nights of pondering his actions for his mind to break. He remembers the bullying, and the hell he was put through, and he decides he would own up to it. He would be the Clown, the Joker. He would quit his job, and start a career as a criminal. He has already killed someone, so he was already going to Hell, what were a few more bodies?
I posted this theory elsewhere on the internet and was accused of hating on religion. While I am admittedly, not a great fan of it, I respect it did have a place in this world. But I chose Catholicism because of the sin of Suicide aspect of it, there are probably other religions with that similar rule, but Catholicism is the one I know about.