People like to demonize this man when he couldn't have been more obvious in that he was fond of Diana but not in love. The "whatever love means" should have been a big clue for her, she chose to ignore it.
People think Charles found Diana amusing, and he was fond of her. But he did not have the spark with her that causes people to fall in love. Intellectually, there was a big gap, due to age and due to personal interests.
In the early years, Prince Charles made a sporting effort to love her as a wife and make the marriage work But Diana had her issues, was jealous and prone to dramatics, and Charles in his stoicism had no idea how to cope with an overly emotional woman.
When Prince of Wales heard of her death he was massively distressed. Some say it was guilt, others that he was only concerned about how this would reflect badly on him. People believe both may be a bit true, but that the majority of his grief was in the thought of the effect on his two sons Prince William and Prince Harry.
Although Prince Charles only realized he loved Diana after she died, according to a friend of the late Princess of Wales. The princess, who died in a car crash in Paris in August 1997, had been divorced from Charles.
Princess Diana also loved Charles her whole life and if circumstances were different they would be together till today.