William never forgot a poignant conversation with his mother when he was a teenager.
One day, Diana told him, he would choose a wife. And the most important thing was she should be his friend first.
With experience born of her own unhappiness, Diana explained that friendship was the most important thing in a marriage – and with it would come enduring love. It was Diana who made Wills the man he is today – caring, compassionate and considerate of his fiancee's feelings.
Her influence was evident in every smile and joke – and in the prince's reassuring pat on Kate's knee – as the couple announced their engagement last week.
'Diana had a close and loving relationship with her sons, with none of the chilliness sometimes found in the older royals,' royal author Robert Jobson says. 'It's clear William looked for the same warmth and found it with Kate.'
As a former kindergarten assistant, the 'Queen of Hearts' was determined her boys would live as normal a life as possible. From the start, she broke with stuffy royal tradition and insisted on being a hands-on mum.
She was the first Windsor parent to take a child with her on tour, flatly refusing to leave her beloved 'Wombat' – as William was nicknamed – behind in Britain when she and Charles toured New Zealand in 1983.
Herself the child of a broken marriage, she vowed her children would never feel abandoned. It was Diana who played with William and Harry, read them stories, splashed with them at bath time and took them to nursery school.
The perfect playmate, she joined in roller-skating along the corridors of Kensington Palace, fun park outings or dressing up as ghosts using bedsheets. 'Diana was very tactile and loving,' a friend told New Idea. 'How lovely to see that Wills' girl is just as warm as his mum was. I think Diana taught him to settle for nothing less.'
Single-mindedly, Diana faced down royal protocol and sent her boys to relatively 'normal' schools. She made sure they got a glimpse of the world outside palace doors.
One Christmas, they stood in line outside London department store Selfridges, to visit Santa. Another festive season, she took William and Harry to the cinemas to see Snow White and the Seven Dwarfs, and then to a fast-food restaurant for hamburgers.
More importantly, Diana passed on her generous spirit, taking her boys to meet the sick, homeless and needy at shelters, schools and hospitals. The prince has carried on his mother's work, becoming involved in causes from conservation to tsunami relief.
Movingly, on becoming royal patron of the Child Bereavement Charity, William spoke about the devastation of losing his mother. She died in a car accident in 1997, the year after her divorce from Prince Charles was finalised.
'I know what it is like to lose someone you love,' he told the audience. 'Never being able to say the word “Mummy” again sounds like a small thing. However, for many, including me, it's now really just a word… not a day goes past without you thinking about the one you have lost.'
Like his mum, he isn't afraid to show his feelings. Once, when Diana was crying in her bathroom, the young prince pushed tissues under the door before coaxing her out for a cuddle.
One very important person will be missing on William and Kate's wedding day. But Diana's words live on through them.
'If you find someone you love in your life,' she said, 'then hang on to that love.'
It’s been lurking in every phone since 2014.