"The only books she ever read were mine and they weren't awfully good for her," Dame Barbara Cartland, her step-grandmother, once said.
Starved of love by her father and mother, the shy teenager had spent years waiting for a real life Prince Charming to sweep her off her feet and to cure all of her ills.
When Charles finally popped the question in early 1981, she formally accepted three weeks later after a holiday in Mollymook, of all places, (her mother Frances Shand Kydd had lived in Australia for years by that point).
They royal couple found themselves on the precipice of marriage for entirely the wrong reasons.
We all know how their particular romance ended, with acrimony, hatred and the infamous "War of the Wales".
But lost in all of that is the creepy story of how they first met, which she revealed during that now-infamous engagement interview.
First, some quick background. In 1977, Prince Charles was dating Lady Sarah Spencer, the beautiful daughter of Earl Spencer, who had grown up at Althorp, the enormous Georgian pile that neighboured his other Sandringham estate. She finally worked up the courage to invite him to Althorp for a shooting weekend. (Ah, there is nothing like killing small birds en masse to really cement a new relationship.)
It was there, walking in a field one day, that he met the woman whose future was inextricably linked to his: Her younger sister, Lady Diana Spencer.
But this is where things get icky.
At the time, Charles was 29, a dashing man-about-town whose romantic conquests were well known in certain, aristo circles.
She, by contrast, was only 16, but was instantly smitten by the future King.
During the couple's 1981 engagement interview, a disturbing fact about that initial meeting came out. Asked about the day, Charles said he recalled thinking "what a very jolly and amusing and attractive 16-year-old she was".
Gross.
He was nearly 30 years old at the time, a man who by that point was sexually experienced, while she was the virginal, teenage, younger sister of his girlfriend and there he was, thinking what an attractive young filly she was.
The idea of an adult man ogling a teenage girl is really uncomfortable.
While nothing happened at that stage, the royal family soon had their eye on her as a potential future bride. (Again, ewwww.)
That same year, Charles invited Diana (along with her sister Sarah) to his 30th birthday ball (she was only 17, mind).
Charles later broke up with Lady Sarah after she committed the cardinal sin of blabbing to the press about their romance. When she finally confessed that a story full of her horrifying quotes was about to come out, royal biographer Tina Brown claims he said to her coldly: "You've just done something extremely stupid."
Invitations for Diana to events where Charles would also be in attendance started to flood in and soon she was asked to come along to a weekend house party which the Prince would also be attending, a situation contrived by Prince Philip and the Queen Mother.
That evening, after a long conversation, Diana said Charles "practically leapt on me" and the rest is sad, troubling, horrible history (with the exception of the births of William and Harry).
Looking back, the idea of a mere teenager being eyed up, not only by a grown man, but also his family, as essentially a modern day broodmare is really uncomfortable.
Luckily some things have changed for the Windsors such as the fact the Princes got to chose their own wives rather than having a cabal of elderly aristocrats picking them out for them.
