What made Christmas so especially depressing for the GI's in fallen angels

The war in VietNam was an entirely different kind of war than any which had ever been fought by US soldiers.  Richie Perry, young, rebellious and impressionable, ends up fighting in Viet Nam, knowing nothing about killing, guerilla warfare, or the atrocities that go along with war, especially the war in an overgrown jungle in which children might be the worst enemy, the Viet Cong.  Christmas in such an environment cannot help but be depressing because they have much difficulty believing in anything so faith-based as the birth of a child nearly 2000 years ago.