Read up on how the WW2 US Army was recruited, trained, and sent into battle. To the degree possible, squads were kept together from boot camp to build up 'unit cohesion' based on sticking up for each other. Most people don't fight for abstract principles, they fight to stand up for their friends, and/or to not look bad in front of their friends. It's bad policy to ignore that.

Soldiers don't make policy.

And yet I find the answer to "Why did you sign up?" to be "To defend the guy next to me!" an odd statement.

Should no one be a soldier in peacetime? Because logically, if the hypothetical guy next to you isn't getting hypothetically shot at...

It isn't why people sign up; it's why they continue to fight after the reasons they signed up have become destroyed in them.