Why was the smile of the soldier boy deceiving?

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.