In 2008, Prince William and Kate Middleton had already been dating for several years. Between 2005 and 2008, William trained at the Royal Military Academy Sandhurst before joining the Household Cavalry. After that, he could have left the military entirely in order to serve as a working royal, which is another reason why many people thought he might choose that time to propose to Kate.
According to one royal author, Kate also thought that William would propose to her at that time - but he instead chose to announce he would be joining the RAF Search and Rescue Force.
"With that single decision, Kate's life was turned upside down," royal author Katie Nicholl wrote in her book Kate: The Future Queen (via OK!). "She had expected a proposal once William had completed his military training, but instead, she was forced to accept that there would be no engagement any time soon. Kate would have to wait even longer."
Katie continued: "Was Kate prepared to join him? She supported his desires and ambitions, but privately she was crushed. William was asking her to be an army wife — without the nuptials. Her frustration was understandable."
The long-distance relationship was hard for Kate, according to the author. Apparently, the Princess saw it as a 'step backwards' and 'her plan to be married with children by the time she was thirty was looking increasingly unlikely'.
But all's well that ends well, since William did propose in 2010 with their royal wedding following shortly after in 2011. And as you surely already know, the two welcomed their three children after this: Prince George in 2013, Princess Charlotte in 2015, and Prince Louis in 2018.
These days, they are considered one of the most powerful and respected couples in the world - so, it all worked out fairly well we'd say!