The evolution of the democratic polis placed great weight on human decision-making. The poleis were acutely aware of being different from other cities, and aware that they had evolved in a revolutionary set of changes from older, aristocratic forms of government. The notion that change could be good was built into the Greek culture. Plato sometimes talks about Egyptian wisdom, but this was considered mainly mystical. There is in fact very little in common between Egyptian thought and classical Greek philosophy.
Greek philosophy is strikingly original.
I don't know about Aristotle, but Alexander certainly had both male and female lovers. A myth is an explanation which has significance for a particular culture. Myths can be true. The myth of Abe Lincoln's rise from a humble log cabin to the White House is, for instance, true, at least in its general outline. Etiological myths are often developed to explain how things came to be the way they are.