The main question discussed between Eun-Pyo and Jung-Yoon in the cave scene from the film "The Front Line" revolves around the ceasefire announcement. Both characters laugh hysterically upon hearing the broadcast that announces the ceasefire, which comes after countless lives have been lost in the line of duty. This moment of dark humor underscores the irony and futility of war, as they have just experienced the devastating consequences of conflict, only for it to end abruptly with an announcement. The laughter can be interpreted as a coping mechanism for the overwhelming emotions they feel in response to the news. The scene is a poignant commentary on the human cost of war and the arbitrary nature of peace. For more detailed insights, you can refer to the film's analysis on TV Tropes.