Much of Fluttershy's life has been fairly miserable: she was sired by a Changeling, her mother's father hates her, she is aware that her very existence broke her home; her mother is insane, and she is basically a remittance mare (paid money to live away from her family to avoid embarassing them). She is extremely eccentric, afraid of most other ponies, and lonely much of the time.
The life of Fluttershy's mother is worse — there was obviously little love in her marriage, she was seduced by a Changeling, bore an illegitimate foal (Fluttershy), her husband figured this out and left her, and she's become a raving paranoid (all the more so because some of her suspicions are true. Unlike Fluttershy, she hasn't made friends or become a mare of destiny — she's just suffered.
