Journey of Embers
Humans are considered by the other races to be a cursed race, being that humans have such a short life span. And any other creature that shares this human blood are considered cursed also because of their shorter life span. Perhaps it is because of this shortened mortality that has allowed humans to better appreciate everyday life.
This appreciation can clearly be seen in their arts. It can be said that the greatest of the arts has been created by Humans whether it be paintings, songs, or literature. This fondness of art might also explain their diplomatic prowess.
It is by no accident that the allied races has been ruled over by a human family for so long. Humans have earned a great deal of respect and have been seen as a race that can be relied on for fair decisions.