Or perhaps she was incapable of appreciating the love given to her, always needing more, then moving on.
Had her father not spoilt her, punished her even for the many spiteful acts (her nannies, her step mother), taught her that tantrums don't always achieve what you want, who knows.
They tend to be marriages where both were in love with each other, not just the title. They also tend to be where both people are prepared to give and take, and really work at it.
To continue the lie Diana told with regard to the Royal Family is rather stale... especially as we have all seen the pictures that contradict that tale, to anyone with a modicum of sense!