Fluttershy seems to have the least amount of background of the mane 6. We don't know her family, we don't know her pastlife outside flight school, and her quirks like The Stare are almost never explained (Though the fact that she can't really control it might suggest that she has slight dissociative identity disorder). Plus, In episodes like Griffon the Brush Off, Crystal Empire Part 2, and Testing Testing 1, 2, 3, Fluttershy seems to repress alot of negative opinions. I'm willing to bet the writers are foreshadowing something about her. I actually think this is what makes Fluttershy a good character, her shy nature and mysterious past allow for a lot of headcannons and speculation.