The book, "War," from Sebastian Junger is well worth a read. I read it before seeing "Restrepo" (which I also recommend). On Goodreads, I wrote this about Junger's book and his take on men and war after finishing it:

"[T]he experience of warfare seems to strike a chord deep within the psyche of the men in combat. The bonding among men, the heroism, the swinging extremes of victory won only by the palpable threat of mortal danger, the adrenaline rush and flood of endorphins during and after battle seem to be almost overwhelming to the human animal, eclipsing any larger moral considerations or even the rest of what we might call everyday life. To the men in the thick of the fight, it's an unrepeatable experience that they find themselves strangely missing years afterwards. What they miss doesn't minimize the horror of what they've experienced, and that's what they find so strange. In some respects once the shooting starts, the soldiers have all the justification they need to continue indefinitely. The immediacy of the experience provides satisfaction enough and requires no further rationale. In some ways it creates a microcosm of life and packs more than a lifetime's worth of living into its fleeting moments. How can life afterwards compare?"