Read this line from The Fall of the House of Usher: Shaking off what must have been a dream, I scanned more narrowly the real aspect of the building. Why does the narrator put the word must in italics?

The narrator more than likely used italics when writing the word "must" to provide emphasis. With the italics, the sentence can be read as if the character just had an experience that they cannot explain or describe in a way that makes sense in reality, and therefore convinces themselves that whatever just happened must have been a dream (On a side note, this is a common way for someone to react to a traumatic experience, or things that seem impossible. It is simply the brain coping with information that seems incomprehensible).