Again using the United Kingdom as an example, provided a princess is a direct descendent of the ruler, she must first obtain permission in order to wed her dearly beloved. And while men today occasionally request permission to wed from the father of their intended, this is still serious business for these royals.
First step: Talk to the ruling monarch and try to win him or her over to the cause. If that route doesn't work and the king or queen disapproves, all is not lost for our petulant princess. She can consult with Parliament to feel out whether its members are all right with the marriage. If both houses have no objections, she simply needs to wait a year, and then she can get hitched to her honey.