The ancient Greeks esteemed many values, which they referred to as virtues. Collectively, these virtues--along with beauty--formed the Ideal Type, or the perfect human. Among these virtues were glory and piety.
The goal of every Greek's life was to earn glory. This was accomplished by living a virtuous life and dying a glorious death. Perhaps the most glorious death was dying in battle in defense of one's country. Thus the Spartans (and other assorted Greeks who fought with them) who died at Thermopylae while attempting to stave off the Persian invasion earned glory.
The Greeks also highly valued piety, which meant revering the gods and giving them due honor. Greek literature is replete with stories of people who prospered by honoring the gods or received punishment for dishonoring them. For example, Homer relates in the Odyssey that Odysseus' men ate the sacred cattle of Helios, the sun-god. As punishment, Zeus destroyed their ships with a storm.