Why did Sparta and Athens unite in the peloponnesian wars?

Sparta was a closed society, and losses of citizens in wars could not be replaced. These losses had been accumulating during the Peloponnese War in the 4th Century, and were compounded in the wars against the Theban confederation at Leuctra in 372 BCE, and Mantinea 362 BCE. As a consequence, as a rural community with a now low warrior strength, it simply didn't have the resources. Although Sparta joined with Arcadia and Achaea to block the pass at Thermopylae to prevent Philip of Macedon's push south, it subsequently decided it could no longer take the field with hostile neighbours Arcadia and Messenia (which Thebes had liberated from Spartan rule) threatening its territory. In consequence Sparta stayed at home as Philip overcame Greece.