... because they haunted his dreams?
If you hadn't notice the protagonist had a shady power that mind controlled people around him including the prince.
What seriously
When was that shown?
In the novel, the explain that, since she was chosen as the prince's fiance, she tried to become the best at everything and while doing so she unknowingly became seen as arrogant, as she just said everything that was on her mind without filter, more or less.
(from now is my assumption)
Thus, the prince's feelings changed completely, that's why, with the push of the heroine, he could be so cruel without remorse
HOW THE ACTUAL FUCK CAN THE PRINCE BE SO CRUEL WHEN THEY WERE GOOD FRIENDS IN THE CHILDHOOD Σ(っ°Д °;)っ