I knew a simple soldier boy
Who grinned at life in empty joy,
Slept soundly through the lonesome dark,
And whistled early with the lark.
In winter trenches, cowed and glum,
With crumps and lice and lack of rum,
He put a bullet through his brain.
No one spoke of him again.
You smug-faced crowds with kindling eye
Who cheer when soldier lads march by,
Sneak home and pray you'll never know
This line might allude to a denial of those who couldn’t handle the pressure of a combat situation. 
 It could also relate to the idea of pain in that remembering those who took their lives too young would be a difficult reminder of the situation that soldiers like Sassoon found themselves in.
Suicide in the Trenches