It's based on the engagement interview, when the couple was asked if they were in love with each other, and Lady Diana was the first to answer, rather shyly, "of course," while Prince Charles said, "whatever it means to be in love." Many of us were taken aback by Charles's words, because we thought that if they were getting married, they must simply be in love.
We didn't know they'd only met thirteen times, and that most of their courtship had taken place over the phone. Thirteen in-person meetings. This doesn't mean dates, like having dinner alone or going to the theater together, but it does mean they met thirteen times. Some people don't experience love at first sight, and Diana hadn't been in love before Prince Charles arrived; so she fell in love or infatuated quickly. Charles didn't.
King Charles had maintained his friendship with Camilla throughout her marriage to Andrew and then to Princess Diana, but Diana and Charles loved each other. King Charles later said he loved each other, after everything that had happened. They were a mismatch, and Diana had mental health issues, which really affected the marriage, so they couldn't find a solution.
Diana, then Princess of Wales, left her marriage for a love affair (sex), and I believe she did so for many reasons. It's not as simple as saying Charles loved Camilla, so Diana left the marriage bed first. I believe she felt depressed, overwhelmed, and most mental illnesses don't manifest until one's 20s. She had issues: abandonment, borderline personality disorder, an eating disorder, postpartum depression, and paranoia. It's a difficult situation for any husband to handle, especially at the time, the early and mid-1980s.
Charles loved Diana. She didn't realize it and looked everywhere for Camilla, even before they had a relationship. So yes, Charles was depressed over the death of his ex-wife, the mother of his two children. He had loved her. He couldn't believe that such a young woman could walk away, leaving him the sole parent of two angry, hurt, and grieving children. It wasn't easy.
A friend told Camilla that Charles wasn't feeling well and asked her to cheer him up. Camilla always knew how to make then-Prince Charles laugh, and she did. They spent time together when he wasn't busy with engagement tours or with his children, who weren't ready to marry Camilla. Not so soon after Diana's death.
Before Diana died, she and Charles were on good terms and were friends. I think they would have loved their children completely, and that Harry wouldn't have had all the problems he has now. It's a tragedy that Diana died and that King Charles must now prioritize the monarchy over his son. But that's the way things are, and wishing it were that way won't change anything. People can only try to move forward in a positive way, changing to become better people.