A father is fitting to give mussar (ethics) to his son. For in his being a father, the frivolousness of youth (yaldut) has left him. Likewise for the mother. They are baalei mussar (exponents of morals), and especially since it is incumbent on a father to guide his son in all matters.

Because this tractate speaks of good and just mussar (ethics), it begins by stating that it is proper to receive mussar from the fathers. And without a doubt, these men are the fathers of the world. For certainly Moses is a father of the world, and so too Joshua who received the torah from Moses, and likewise the Elders, etc... and so too Antigonus and the other sages mentioned.. They are certainly fathers of the world (i.e. of all humanity). Thus, it is proper for one to accept their mussar just like a son should accept the mussar of his father. And it is proper for them to give mussar to the world, since they are fathers of the world. Therefore this tractate is called Tractate Avot (Fathers). For it contains the mussar teachings of the fathers of the world. (end quote)

"Moses received the Torah from Sinai" - since this tractate is not coming to explain a mitzva (commandment) in the torah like the other tractates. Rather, it is all ethics and character traits, and the wise men of the nations also conjured up books of ethics according to their hearts on how a person should live and behave towards others. Therefore, the Tanna (sage) begins this tractate "Moshe received the Torah from Sinai". To teach that the traits and ethics in this tractate were not conjured up by the sages according to their hearts. But rather, even these were said at Sinai. (end quote)