Why were the Spartan soldier's willing to sacrifice themselves at Thermopylae?

Answer 1

When the Greek force holding the pass against the Persian invasion was withdrawn after their mission of temporary delay was completed, the Spartans and Thespians remained behind to allow them to escape before the Persian cavalry broke out and rode them down the open country. They held the pass for an extra few hours to achieve this, selflessly sacrificing themselves for the other Greek city contingents.

Answer 2

The Spartans were part of a very militaristic society. Every day, and every activity reflected their values in becoming the best soldiers. They trained from childhood throughout most of their lives to be at the ready if Sparta needed to be defended. They were taught and believed in self-sacrifice to the good of the state, and while they sacrificed daily through their extreme training, they believed that the ultimate sacrifice they could give to the state, to Sparta, was their lives. They believed the most honorable death was a death on the battlefield fighting for Sparta. To retreat was not even a thought that crossed their minds because of how devoted they were to this lifestyle. In addition, their last stand at Thermopylae was necessary for the other Greeks present at Thermopylae to successfully escape (and yes, there were roughly 5,000 Greeks at Thermopylae, the movie isn't entirely accurate, although there were only 300 Spartans).