Sparta and Athens were two of the most powerful state-cities in Ancient Greece. And they were different in every sense. While Athens was built on democracy, Sparta was built on military service.
Spartan men devoted their lives to the military service. They lived communally well into adulthood. Spartans were taught that loyalty to the state came before everything else. A Spartan would rather die on the field than come back home.
Spartans existence is similar to being a monk. Your room is bare, you live simply, and eat sparingly. Boys were even encouraged to steal food so that they learn skills they can use on the military field.
Spartan men had to stay fit and be ready to fight until the age of 60. And all of that stems from the fact they considered to be direct descendants of the Greek hero Hercules.