A large part of what makes the Dark Knight such a compelling story is the dynamic between Batman and his perfect opposition – the Joker. A large part of what makes the conflict between the two so riveting is that the Joker is amazing at targetting Batman’s weaknesses (which he wasn’t even aware that he had). The closest thing to a superpower that Batman has is his ability to scare his opponents (Bruce Wayne chose the symbol of the bat as he has a fear of them). The Joker appears to have no fear, making it difficult for Batman to fight him. The Joker is delighted when Batman fights him physically, evening wanting Batman to kill him (turning Batman’s moral code into a weakness).
The Joker also pressures Batman into difficult choices (whether or not to reveal his identity and whether he should rescue Harvey Dent or Rachel) in an attempt to reduce him to his most desperate state and pushing him into revealing his true character. Batman’s unwillingness to reveal himself and his inability to stop the Joker’s destruction turns public opinion against him. With his reputation in shambles Batman is ready to give up and fold to the Joker’s demands. Harvey’s intervention is the only thing that stops him from turning himself in.
Another thing that makes the Joker so compelling is that his goals mirror Batman’s, bringing the two of them into direct conflict. Batman fights to create a Gotham without crime. The Joker wants chaos, wanting the civilians to show their own ‘true colours’. The very nature of their conflict means that only one of them can win, which is a daunting propect as the Joker has just as much power as Batman. The Joker forces Batman to question all that he thought he knew about criminals and vigilantism (Batman previously believed that the root of Gotham’s crime was money). Batman has to learn to stop underestimating his opponents (and therefore stop overestimating his own abilities) due to his previous strengths becoming weaknesses.
In the competition for ‘the soul of Gotham’, Batman learns from the Joker.
Batman becomes the Dark Knight because of the Joker.