People have real flaws so why don’t characters?
What all of this means is that readers will put the book down if they don’t identify with, sympathize with, or want to be like, characters in the book. If the reader has no emotional investment in any of the people in the book, then they don’t give a crap about what happens to your protagonist. They won’t want to see the antagonist get his (or hers) in the end. They won’t care whether the love interest blossoms into a relationship. They simply won’t care – no matter how good the plot is. You can have a fantastic plot driven by flat, unbelievable, caricatures (not characters).