Pinkie Pie is an orphan and spent at least part of her childhood in an orphanage. She mentions that her parents were killed in an untimely manner, likely by the fire that ravaged her home town. Shortly after this event, she was moved to the orphanage, where she was taught a variety of songs. Applejack describes them as "Sad...with a deceptively happy tune."
Both her experiences as an orphan and as a grown-up suggest she has had to live with prejudice all her life because she is an Earth pony. Pinkie Pie is often cynical, and views herself as "unworthy." There is some truth to this, based on the apparent racism or favoritism that is present in the creatures of Equestria. Even the dragon in the third episode is not averse to adopting a pony, but will only take a Pegasus or a Unicorn.
Pinkie Pie is peppy and cheerful on the surface, but her troubled past had left severe mental scarring. She is especially sensitive to abandonment and rejection. She does find some solace in singing.
Lack of a happy childhood has made Pinkie Pie often take part in childish activities such as trick-or-treating at the Molest Fest, in order to derive some happiness from what she originally missed out on.
Relationship with the other poniesEdit
The other ponies make an effort to ignore Pinkie Pie as much as possible. They view her antics as strange and undeserving of attention.
Pinkie Pie is often accused of being a Gypsy and performing "Gypsy magic", which is seemingly unrelated to Unicorn magic and may have its roots in potions. For instance, she and her "gypsy magic" are blamed for the chaos caused by Fluttershy's parasprite in "The Perfect Swarm". When accused of this, Pinkie was adamant that she was not a gypsy and knew nothing about magic, being an Earth pony.
However, Pinkie later developed some potion-making skills by consulting a nearby witch doctor. In "Cute From The Hip", she attempts to open a portal by making a special concoction so she can travel back in time and save her parents from dying. There is evidence that she may be further using this portal-creation ability to somehow oppose Fluttershy's cult, as referenced in "Neigh, Soul Sister".
In "Cherry Bomb," Pinkie Pie announces that she has begun to embrace her "inner gypsy" and is getting back in touch with her Gypsy heritage. She learns to talk to seemingly inanimate objects, notably her magic candy.