"That morning brought the Crown Prince a sense of regret and utter confusion. He and the First Princess were unaware of what brought them into this position but the First Princess's heart wasn't filled with regret unlike the Crown Prince.
He left her with a promise that he would be back for her. The Crown Prince had an inkling that his King could be the one behind this and he was proven correct. It broke him to think that he was being used in this way to destroy the life of an innocent person.
Determined to not let anything happy to the First Princess, the Crown Prince tried everything that he could but he failed and then he had to accept the reality. It was a choice that the fate presented and the Crown Prince chose to let the situation break him.