Why did this happen?

Scarlet heart X The man who sells the river

Minseok is a slave who works at the palace in Goryeo. He's friends with Yoo Seungho and Ko Changseok. They want to leave the palace but that's hard to achieve. Even if they manage to escape, they had nothing. So they needed money. They make plans to rob the king. They set Minseok up to get close to one of the princes, in this case, the 10th prince. Byun Baekhyun. They want him to befriend him, let Baekhyun basically fall in love with Minseok so he trusts him enough, so Minseok could break his heart after and they had all the money.

But breaking the heart of someone you actually love is something Minseok didn't wanted to do.