Moses received the Torah from Sinai and handed it to Joshua, and Joshua to the elders, and the elders to the prophets, and the prophets handed it down to the Men of the Great Assembly.
Why do we start here? This is a book of ethics. Why do we bother with this chain of transmission? Why not just give us a list of dos and don'ts?
Let's begin at the beginning of the chain. Moses received the Torah from Sinai. Rabbi Ovadia from Bartenura (1450-1510 CE) explains that... (I'm paraphrasing)
The wise of the nations of the world also have books of ethics and character, how a man should deal with his neighbor - it is because of this that our Pirkei Avot start in this way, "Moshe received the Torah from Sinai" in order to say to you that these ethics and character traits are not assembled and written by wise men who came up with these rules from their own hearts... rather these ethics and character traits were given at Sinai.
These ethical rules do not change from person to person or from generation to generation. These are hard and fast rules that we received at Sinai as part of the Torah itself. This is why it is important to begin where it does. These ethical rules are not relative or subjective.
We have seen cultures and societies where the old and infirm are killed because they are a "drain on society". We've seen societies where the wives of those who have died are killed in order for them to join the dead in the afterworld. When you have subjective ethics... all this is allowed. When you have objective ethics given at Sinai by G-d... none of this is allowed.
Ethics are as important as rule of law. A society without true objective ethics is one where the law has no restraint and all is permissible. A scary thought.