Fio is constantly disappointed in herself, constantly berating herself. She learned it at her mother's knee. Her parents were concerned that she not have a swelled head--think too highly of herself. There was also probably a bit of the protective "don't reach too far or you'll get your arm cut off" involved.
Dad always said praise never helped anyone, that he wanted criticism (although Fio never saw him take it well). Maybe that's why when Fiorella made an impossible 98 1/2 on Miss Osborn's Latin test, he smiled and asked,"Why didn't you make 100?" Mom, whose mantra was that one shouldn't praise kids for doing what they should do anyway, was more subtle, challenging Fiorella's confidence with comments like "You just THINK so," when Fiorella said she could tell whether Baby needed a diaper change or not. And questioning the price Fio paid for engine work on her car, when she herself didn't even hold a driver's license.
Yes, the practice drove Fio to strive harder, to achieve more, but it has also left her unhappy with herself. And now she feels bad because she's criticizing her parents, who loved her very much.