What does it mean that the men use "a hard vocabulary to contain the terrible softness"? why is it terrible to be internally soft?

This line comes from "The Things They Carried" by Tim O Brien when he served in Vietnam. The term hard vocabulary to contain terrible softness simply reflects what he has done to serve in Vietnam. In times of war, it is not good to be internally soft as it may serve as weakness to enemies. Therefore, hard vocabulary refers to the words that one would say to himself and others in order to project strength and hide softness or weakness inside.