"As Toilsome I Wander'd Virginia's Woods"

I've been reading a few more poems in the Drum-Taps section, specifically, "Vigil Strange I Kept on the Field One Night," "A March in the Ranks Hard-Prest, and the Road Unknown," "A Sight in Camp in the Daybreak Gray and Din," and "As Toilsome I Wander'd Virginia's Woods." All of these poems make me sad, except the last really touches my heart. It's not that it's not sad, because it is sad, I think it's because Whitman reminds me how deeply things affect us. In this poem he sets the scene in Autumn, listening to the rustling leaves. The speaker of the poem has quickly marked the grave of a soldier at the base of a tree. He scrawls the words, "Bold, cautious, true, and my loving comrade." Then the poem moves forward to another time and place, he says:

"Many a changeful season to follow, and many a scene of life, / Yet at times through changeful season and scene, abrupt, alone, or in the crowded street, / Comes before me the unknown soldier's grave, comes the inscription rude in Virginia's woods, / Bold, cautious, true, and my loving comrade." 

This moment follows the soldier around forever and hits him at any given moment. I think Whitman was struck by the emotion of the situation. He wrote with such detail, I feel like I am standing near the tree watching everything unfold. Both soldiers have become a tragedy of war. 

War is a tough subject to write about. It's sad, destructive, and ugly. There is always a romantic ideal: democracy, love, or some other reasoning. As I read these poems I feel it must have been difficult for Whitman to write about the sadness. It must have had a staggering emotional toll. I wonder what kept him going? Was it sheer madness or willpower?