Why would two men fighting in the same army become fearful of each other?

Two men in an army are supposed to be comrades and have one another's backs, but there are situations where this can fall apart. The collection The Things They Carried explores this idea to some extent.
One reason men would become fearful of one another is if they got into a legitimate confrontation. During war times, tensions are extremely high, and men can end up getting violent. Fights can break out between soldiers on the same side, but they typically end up resolved with just a scuffle. Additionally, a man can become fearful of another if he goes too far in his actions against someone else (even an enemy).
The book, however, explores a different idea. Lt. Cross is the leader of the men, but he is far too focused on a girl back home. In the end of the novel, he resolves to forget about her and become a better leader, but this is a situation in which comrades can become fearful of one another—if they don't trust their leader to take care of them or be focused on the task at hand.