Possibly no greater explanation can be given for the many origins of the Joker, and simultaneously Batman, than the story written out by Neil Gaiman in his graphic novel titled "Batman: Whatever Happened To The Caped Crusader?"
In this unorthodox view of the life and death of the Dark Knight it explains that Bruce Wayne actually does have a super power. That power is reincarnation. Gaiman explains how whenever Batman dies he is reborn as himself with the eternal destiny to become the Batman. However, what this looks like can vary greatly. In one life Batman might be the crime fighter we all know and love. In another he is a mutated bat monster feeding off the blood of his victims. In yet another he is a television actor simply trying to make it big with his new role.
There is however one reality which shows how the Batman will always be followed by the joker in some form. Gaiman shows a world where Bruce Wayne is left without the ability to cope with his parents death, except through the elaborate game created by his concerned butler Alfred.
Alfred is shown to originally have a history as a traveling theatrical performer. He later finds himself in the employ of the Wayne family, whom he becomes fond of serving. After Bruce's parent were murdered he naturally began his regular destiny by putting on a mask and attempting to fight the criminals of Gotham as a way to fight against his parents murder.
"I found him when he fell, and sewed up his wounds. And then he began dressing as a bat... As an Englishman, I find it difficult to identify the place that eccentricity ends and madness begins. That Master Bruce was eccentric, I do not deny." -Alfred Pennyworth
Alfred then decides to create a fantastic world of fantasy to help Bruce feel like what he did had meaning. Instead of fighting the real criminals of Gotham Bruce would simply be playing a role in an elaborate play. The protagonist being his self-made role of Batman, and his first foe the Joker.
"What Master Bruce needed was a Moby Dick for his Ahab, a Moriarty to his Holmes. And so, regretfully, I did what needed to be done. White greasepaint. Red lipstick. A purple suit. A green wig. And it did nothing, until I smiled..." -Alfred Pennyworth
Alfred himself was the Dark Knights mortal foe. So perhaps while Bruce Wayne is always the constant, Batman and the Joker are nothing more than reflections of the unsteady balance in the world shown through the mirror of one man. For some people good and evil might be a battle fought between two men. For others it is a struggle between differing worldly ideals. Or it could be the ethical and moral battle being fought within ones own soul. Either way, at any point in time there is always the Batman opposing the Joker, and the battle will forever be destined to repeat itself.