Ancient Greece was quite unique in the ancient world (and by this, I understand ancient China and India too), because
1) it developed a culture of independent, self-governing city states with a multitude of constitutions ranging from oligarchy to democracy; AFAIK, besides Greece, Rome and Carthage, no major ancient civilization developed strong republican structures: instead, Persia, Mesopotamia, Egypt, China and Central Asia were ruled by monarchs, often backed by priestly caste
and
2) Greece never really united under one leading power. Egypt was united pretty fast, China had large kingdoms fighting each other before being conquered by Qin, Mesopotamia was subject to different empires, as was Persia. Greece however stayed a mass of poleis, hostile to each other, resisted against Macedonian domination and only came to rest after Rome formed the province of Achaia.
So we have to quite unique features of Greece: its political diversity and its fragmentation. How do we explain these features? Sure, the fragmentation certainly influenced the diversity, and the diversity strengthened the fragmentation. However, that doesn't explain why Greece stayed disunited while Italy was unified under Rome, even if both Italy and Greece consisted mainly of city states.
I'm interested in how you explain both phenomena.