The first reason is that it was stated early on in the story that the king loved his daughter above all and she was the apple of his eyes. This led me to think that the princess may be spoiled and willful. She might be used to getting her way by any means necessary. This actually confirmed later in the story when it says that the princess used gold and her womanly wills to get the secret of the doors.
The second reason is that the princess comes from a long line of hot blooded barbaric descendent. And since breeding can play a big part in our ideas