Before Grossman proceeds with the Burden of Killing he ends this train of thought with a metaphor:
Many authorities speak and write of emotional stamina on the batllefield as a finite resource. I have termed this the Well of Fortitude. Faced with the soldier's encounters with horror, guilt, fear, exhaustion, and hate, each man draws steadily from his own private reservoir of inner strength and fortitude until finally the well runs dry.
Not just individuals but units as a whole has this Well, and sometimes green ones can perform better then veterans as they have more Fortitude to spare.
One can replenish from the well of others, for example a good leader can inspire his subordinates and help them holding on longer. Victory and success also helps the individual and the unit to go on.

Now I'm gonna end this for today too. I'm not sure if my editing is adequate and clear which lines are the quotes, if something not clear just write.