It is well known that Prince William waited years (opens in new tab) before proposing to his longtime girlfriend, Kate Middleton (opens in new tab). Of course, the engagement would have been an entirely happy event, but apparently Kate had one major regret about this milestone.

In her book "Kate: The Future Queen" (opens in new tab), royal expert and author Katie Nichol explains why this joyous occasion was tinged with sadness for Kate.

First, the backstory: on October 20, 2010, Will popped the question at Lake Lutundu in Kenya (opens in new tab); the two were overjoyed, of course, but decided to wait two weeks before going public on November 3. In the meantime, they did not even tell most of their friends and family for fear that the newlyweds would leak the news too soon. This is where regret arises.

Between the engagement and the announcement, Kate's last (open in new tab) living grandparent, her grandfather Peter, passed away on November 2, just one day before she was scheduled to share the happy news with him.

"Kate was deeply saddened that she did not have the opportunity to tell her grandfather that she was engaged to William," Nicole wrote in the Mirror (opens in new tab)."

"She knew that her grandfather would be happy for her.

Will attended the funeral with Kate, and as a result the couple delayed the announcement, finally telling their loved ones the good news on November 16, hours before the news became public.