The narrator can sound trustworthy because the story has denied everyone else a microphone.

A voice sounds reliable partly because the structure has silenced everyone who might contradict it. There are no competing accounts inside the story, so the narrator’s version stands unopposed and reads as truth rather than as one perspective. Credibility can be a function of who else was allowed to speak.

So when a single account seems compelling, ask who is missing from it. The colleague described in a story, the ex-partner, the departed employee, all had accounts nobody has heard. That does not mean the version you have is false. It means you have heard one microphone and should hold your conclusion accordingly.