Where did this suddenly change of heart come from???

Idk. But maybe this person never had as much love as they ever wanted, so someone showing their undying love to them (physically, emotionally, and mentally) has entrapped them?

We have to wait until next chapter but I feel like now the mc is playing with him trying to manipulate him

Probably understood that this is his best chance to get power over the king and he seems to have accepted the reality first but probably not in love regardless.