Why are Marines so worried about honor and the code?

I get wanting to save and protect fellow Marines in combat, but why volunteer for combat in the first place? Childers says that he volunteered to join the infantry so he could lead Marines into battle, isn't that a horrible reason to join the military? Basically it means that he joined the military so he could fight, no matter what the reason, right? How was shooting at Vietnamese protecting Americans in the USA?

I am so smart, the only thing I can't comprehend is how stupid everyone else is