The poem " A Small pain in my chest " symbolises the agony of the soldiers on the battlefield, the physical , mental and psycological agony.A small pain in the soldier's chest is a big pain in the conscience of humanity.The soldier here, asks for a sip of water, but infact he is actually asking for some concern and understanding for the human being who suffers without any fault of his. Some other examples of the symbolism are:
The sun here is a source of life, the bright sun-shine gives life.But the soldier is feeling cold and the sun is becoming dim for him even in the morning, which symbolises that darkness is entering into his life and death is approaching.
His smile shows that he is grateful  for the help he received from the narrator.
The large pain in the narrator's heart , symbolises the greater guilt on the part of humanity at not being able to give up wars.
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