Self image 

Throughout the series it is made known that Zuko needs to restore his honor, In order to come back to his home the fire nation. No one can imagine the impact of being publicly banished from the only place you've known as home. The theme behind Zuko's banishment comes from him speaking out against a plan to over rule an earth kingdom territory. In result of him speaking out he is challenged to a Agni Kai by his father. Which is a traditional fire bender duel that is centuries old. The aftermaths of the Agni Kai results in Zuko getting a scare on his face and banished from his home. This in turn causes him do everything possible to catch the avatar restore his honor. We can see bits and pieces of how his banishment affected him mentally. For the majority of the series Zuko can be seen at war with himself from the traumatic event that unfolded.