To separate the king and queen after their wedding, so that someone could send him false reports of her having given birth to puppies, or to give her a chance to open the forbidden door, and the like. Or, occassionly, to give the prince or princesses chances to do things not under their father's eye.
To give the prince a chance to save the realm.
To make a queen, or princess, or prince, have to flee the realm.
The fun part is that writing it up in a story based on them, even if you are putting it for the same reasons, you need to develop it more, since none of those are reasons for someone to actually go to war.