In the poem ‘Small Pain in My Chest’, the smile of the soldier boy was deceiving. The speaker especially mentions his smile in two occasions —

“I wonder if you’d help me, sir.”, he smiled as best he could.
And smiled a smile that was, I think, the brightest that I’ve seen.

But behind his bright smile he was trying to hide his pain. Though he repeatedly calls his injury ‘a small pain’, actually it was a fatal injury to which he finally succumbed. He was probably regretful for not winning the battle in spite of being a ‘man of vim and gest’, as he thought himself to be. That is why he needed to hide his helplessness behind his smile.