I agree with everyone, but it was also nice and warm. Greece has a lovely climate, so the people were free from figthing the elements to spend time on more cultural pursuits. The land was fertile and farming was easy. They generally had time for an easygoing lifestyle.
As #4 points out, the plethora (based etymologically on a Greek word) of Greek islands necessitated the advancement of the technical abilities of boatbuilding and shipbuilding, which became incredibly important during their time of cultural hegemony. Being conveniently located on the Mediterranean was likewise a key factor that gave Greeks easy access to other territories so that it could expand its influence and borders.
Because Greece was located on a collection of islands, the Greeks became expert boat builders and sailors. They realized that different designs of vessels were better suited for different purposes and had unique ships for use in trading with other countries and for purposes of war. The geography of Greece was critically important in bringing about these developments.
The mountainous terrain and the abundance of islands (including more than 200 that are presently inhabited) allowed the overpopulated mainland areas of Greece to expand during ancient times. The further expansion into the Italian peninsula and Asia Minor created an even greater mix of cultures that added to the dominance of Greek civilization.
The idea most commonly cited is that the mountainous nature of Greece led to the creation of the poleis. Because Greece is so fragmented by mountains and valleys, it was very difficult for one dominant power to arise. This allowed the rise of many little city-states, each with their own form of government.