What pulled the Greek city-states apart?

Although the Greek city states all spoke the same language and worshipped the same gods, each polis had their own government systems and values. Athens and Sparta, in particular had opposing values; Athens had a democracy, Sparta had a monarchy, where Athenians prized new ideas, the Spartans stubbornly clung on to the traditional etc. Partially due to Pericles taking advantage of the Delian League to benefit Athens, other city states began to resent Athens and eventually the Greek city states sided with either Sparta or Athens in the Peloponnesian War.