The singer will be attending the royal wedding this Friday and insisted that the late Princess would be happy about his choice.
John admitted that the upcoming wedding will bring back memories of his friend's funeral 14 years ago.
During an interview on Good Morning America, he said: "The last time we were in Westminster Abbey, my heart sank when I saw those two young boys (Princes William and Harry) walking behind the coffin.
"I thought that was the hardest thing - I can't imagine at that young age having to walk in public, following your mother's coffin. And the next time we're in the Abbey, it's to see [William] walking up the aisle with a beautiful woman and the love of his life.
"I think it's the most joyous result and I'm sure Diana would be very, very happy about it... I think that she would be very happy with his choice. [Middleton] seems such a great girl, and they seem so much in love."
John added that his only concern for the couple is the media.
He said: "Enjoy it while you can because the press are going to give you a bumpy ride. They are going to love you. They are going to hate you. I just hope they love each other enough to stay away from all that.
"There's one thing in the world if I could get rid of, it's that negativity of the British press - that hatefulness, that horrible nastiness that we have... Yeah [the wedding] is a fairytale thing and people criticise it and say we're making too much of it. But it's about love, and in this day and age anything that's about love and positivity is a welcome relief from what's going on in the world."
