"Why Do Soldiers Fight....?"
This has been studied a lot, and the most common answer I've heard and read is "For my buddies...".
The strong bond between soldiers in a combat zone makes it your ultimate goal to make sure that your friends are as safe as they can, even if it can mean your life...
Here's and interesting article from About.com on the subject:
(if you can find it between all of the ads....)
http://usmilitary.about.com/cs/army/a/soldiersfight.htm