One of Aang's greatest journies, and struggles, was the one to control and master the Avatar State. Where at first it came randomly, or in times of extreme stress, Roku warned that it needed to be mastered in order to save the world.
Enter Guru Pathik, a curious man who knows he can help Aang. Pathik is patient beyond belief, even one time waiting for days before going to Appa to give him a message.
Once Pathik met with Aang, he began his Chakra training and began to help him master the Avatar State. Unfortunately, Aang hesitance to unlock one of his Chakra's caused a rift between the two. As Aang didn't want to live with Katara, who was tied to one of the Chakra's. When trouble occurred, Aang left before his training was complete, much to the dismay of Pathik