Once Hanumanji decided to find the right teacher for himself and decided to approach Sun for the same, since Sun has been looking over this universe since the creation and has observed everything.
He asked Son to be his teacher, but Son told him that he is busy roaming around and does not have time.
Hanumanji found a solution to this problem. He suggested that he will fly along with his carriage, while he can teach him while continuing roaming.
In the end when Hanumanji wanted to offer gurudakshina to the Sun, He asked Hanumanji to always protect His son Sugreev.