Why did the Spartans place so much stress on military service?

There are few reasons why so here are some of them,

They were afraid that the helots might someday rebel, so Sparta made the government force all boys and men to fight in war.

Sparta wanted to make sure that they weren't going to lose any war. Also to make their people tough and strong for wars, because they didn't want anything bad to happen to their slaves.

So that can know that their military is much stringer, and they will not have to use as many people as other countries.