According to legend, long before our time there were two dragons. A red dragon, that embodied chaos, and a black dragon, that embodied the essence of order. The two dragons were originally at odds with each other, each attempting to create their own societies. The Dragon of Chaos in the North, and the Dragon of Order in the South. In the North, the Red Dragon created society after society, but each time, the inhabitants destroyed each other, as they lacked restraint. In the South, the Black dragon too was failing. Where the inhabitants of the North lacked restraint, the inhabitants of the South lacked the passion and ambition nessecary to flourish.
Eventually, the Black dragon saw the need for balance and attempted to reconcile with the Red dragon, but the red dragon was blinded by the chaos in its own mind, and attacked the black dragon, convinced she was attempting to trick and destory him. When the Red Dragon attacked, the Black Dragon managed to use his own chi against him. You see, the Black Dragon had learned to bend the positive jing energy flowing in and around the Red Dragon. The patient Black Dragon waited until the Red Dragon's focus wavered for just a moment, and merged her own chi with his. In that moment, balance was brought between the two forces, bringing about what we now know as fire bending.
It is for this reason that almost every fire bender is born with these two dragons inside of them. The key to become a true fire bending master is to learn to balance the two forces, chaos and order, inside of yourself.
"Almost?"
It is said that every 100 years, the Red and Black dragon return to each other, in the mortal realm, taking the form of two seperate humans. When this happens, it is imperative that the two find each other, so that the Black dragon may balance the chaotic ambition of the Red dragon, allowing for a new era of peace and prosperity for the Fire Nation.