In retaliation for their ill-treatment of her, the evil fairy cursed the king and queen's son to become whatever those who loved him wanted him to be.
The king thought that this was a blessing: his son would grow up to be a great warrior, just like him!
The queen knew better.
Their child was perfect. He never had any other choice. As a baby he never cried; as a young boy he was never willful or contrary (except: the king wanted a son who stood up for himself. The queen wanted one who would be thoughtful and clever. A contradiction).
When he was only their son things were easy. Not perfect, but easy.
But he grew up, as children do, and they couldn't keep him locked away forever. He wasn't a princess, after all! He had tutors and playmates and met the king's court, and it's so easy to love a little prince.
The curse flowered within him, countless grasping tugging his soul back and forth, gnawing away at the little thing's never-formed sense of self like children squabbling over a new toy.
And so the prince learned that love is a selfish thing, and he learned to fear it.