sometimes a book is 5 star because its a masterpiece. sometimes its a 5 star because it made my brain release serotonin. and sometimes its a 5 star because it made me cry my eyes out. all of them are equally valid.
there is something incredibly satisfying about reading the first page of a book, and immediately something in your brain sits up and goes 'oh, i'm going to like this' — and then every subsequent page proves you right.