How does the mood of a story affect the reader?

It operates and change the way they look at a story.

I would say that the mood of a story affects how the reader perceives a story, and what they take from it. When a story has a somber, sincere tone, most readers will take the story more seriously, and pay closer attention to detail, as well as feel empathetic, or melancholic. When a story is more lighthearted and easier to read, I think a reader is more likely to pay attention to rhythm, and diction, and maybe appreciate the satirical aspect of the work.