Why does Jenkin’s death hit Perry so hard?

Richie Perry watches his comrade Jenkins die after stepping on a landmine during his first patrol. It's a very traumatic experience for Richie. Jenkins is a member of the same squad, and like Richie he is still a very young man. Richie's better than Jenkins at hiding his fear; Jenkins always comes across as nervous and frightened. But Richie's still afraid all the same. Jenkins's death affects everyone badly, but Richie's especially hard hit. He wants to write home about the incident, but can't bring himself to do so.
Richie realizes that it could easily have been him who was killed by the landmine. He was out on the same night patrol, he was scared stiff and could easily have made a false step, and he was perilously close to the mine when Jenkins stepped on it. The whole terrifying experience has heightened Richie's sense of mortality in the most brutal way imaginable. No wonder he's been so badly affected by this tragic event.