I think she was looking for love and stability. She was deeply traumatized by her parents' acrimonious divorce and thought that by marrying the PoW nothing similar would happen to her. That's at least what I understand (and I do understand - my parents divorced and battled for years when I was a girl and it has left me with a deep seated fear of being left, being not good enough).
She also seems to have had a girlish ambition to marry a Prince, to marry the highest prize, but also the man who would give her a position where she could unfold her unique talent. I read somewhere that already as a girl she went to visit old and sick people and had rapport with them. She may have had an intuitive sense of being the right woman for the role she was marrying into. She had more than only the Spencer closeness to royalty - she had royal charisma of her own, and this charisma may have needed an outlet, like every real talent needs.
They were both under the illusion of being in love, and at the same time enchanted with what they saw in each other. There are pictures with a real glow from the start of the relationship.
That unfortunate remark "whatever being in love means" sounds ominous in retrospect but may simply have been said in embarrassment - no talks about feelings please. He may have felt that the question was too intrusive, too touchy-feely, and rebuked the interviewer - noticing only much later that he rebuked his bride-to-be with his remark, too. I may be totally wrong but that's my impression re-watching the interview. He may have wanted to make Diana laugh a bit because she was quite tense. I don't know and can't know.
There are always chains of misunderstandings when alienation sets in. What a pity. Their marriage may have worked, I don't believe it was doomed from the start. Charles would have needed more emotional maturity, more willingness to open up. Had he given Diana the unconditional love she craved...
If, if, if.
I'm a few years younger than her and remember my mother saying on the wedding day of Diana and Charles: you will grow up and grow old with her. She'll be the sign post of your generation like Grace Kelly was for me. When I saw Princess Grace I saw my own life, just a few years on.
This has connected me and probably many of my generation to her. Somehow, even after her death and even after recognizing how deeply troubled she was (and how in some of her interviews she molds her memories a bit), she has the power to reach out and evoke emotions.
For Charles, she was probably too much - he didn't expect her to be such a powerful presence with global appeal. I'm certain when he looks back he recognizes what he could have done better. Like we all do.
I like seeing him relaxed and happy with Camilla. I would have wished similar happiness for his ex-wife. She certainly would have deserved it. Had she remained alive, they'd probably all get along well nowadays, thanks to a healthy sense of humour in the persons concerned, and thanks to the shared bonds with the sons.
Sorry for the rant - just my thoughts, I have no way of knowing whether I'm right.