Maybe it just speaks to your subconscious directly? And after finishing and reflexing on it for a bit you might develop reasonable explanation.

It's one of my all times favourites and i had never felt anxiety while reading it. Maybe sudden realization that you are that good of a person provoked by empathy to MC may trigger anxiety? Because I certainly had this kind of empathy amd it is the exact reason i like the book so much. By pointing out flaws it literally makes you a better person. On top of it there is decent anime adaptation in the series called Aoi Bungaku Series