As a young woman, Queen Camilla turned Charles down and married someone else because, like many women she did not want to be a member of the Royal family because, she did not want to live in a gold fish bowl. I think she was one of many of King Charles, and Prince Harry's girlfriends who thought the same and did a runner
Princess Diana, was very young and had, what I always thought, was a crush on Prince Charles, as he was, and did not have the maturity to think through what marrying him entailed.
We had a lucky escape with Edward VII and Princess Margaret's decision has to be seen in the context of the age she lived in. Setting aside family concrens, she would have been excluded from many sectors of society. There were whole sectors of social life where a divorce and remarriage was entirely unaccepteble. Remember Princess Margaret was a practising member of the Church of England.