Zuko was always a gifted bender, he was just an over-reactive, angry little shrew 2/3′s of the time. The dragons showed him the true source of fire and the element he had already mastered took on new life for him.
Let’s not forget to mention how much confidence helps. Zuko was consumed with uncertainty, inadequacy, and frustration for most of the show, but whenever he was determined and swept the negativity away, he really shined. Zuko was always willing to fight Azula, but it wasn’t until season 3 that he believed he could win.