The clue is in a part of the dialogue you have not quoted here. Lower down: "Some of them do (see). Some are very delicate and feel worse than any of us."
Seeing is to be delicate and sensitive and feeling the implications of the situation - the death, the injuries (mental and physical), the love and the heartbreaks, human goodness and human evil, human strength and human frailty. Officers command men and send them to battle - some to their death.
Part of the seeing and feeling is for human relationships. For Hemingway the war is horrible but it is also fertile soil for relationships. The war puts people together. They face pain and suffering together. They form relationships.
"Seeing" includes seeing the consequences of "arms", and therefore the meaning of a desire to say farewell to arms.