* The experience of war: He has already witnessed the horrors of war firsthand, seeing his friends wounded and killed. The experience has left him disillusioned and deeply traumatized.
* His own mortality: He is aware of his own vulnerability and fears the possibility of death or severe injury. This fear is intensified by the fact that he has fallen in love with Catherine Barkley.
* The futility of war: He questions the purpose of the war and the sacrifices being made. He feels that the war is pointless and that he is participating in something that has no real value.
* His relationship with Catherine: He wants to live a peaceful life with Catherine, free from the horrors of war. He worries that returning to the front line will jeopardize their future together.
The narrator is also haunted by the memory of a particular incident where he watched a young soldier die in his arms. This experience has deeply affected him and has made him question his ability to cope with the emotional and physical toll of war.
Overall, the narrator's fear of returning to the front line stems from a complex mixture of personal experience, fear of death, disillusionment with the war, and a desire for a peaceful life with Catherine.