beta
100% black hair is spiteful for some reason and forces mc from the past to marry him so white hair can't have him. But not only that we see they have a hidden relationship so possibly black hair cursed mc so he can't be with anyone.
So here comes my wild hypothetical theory.
The reason why mc need to be with white hair 24/7 is that he possessed the future version of black hair so they can finally be together and by doing so he counters the curse in a way but once apart black hair wants body back and forces not only him but as well affects current curse making up both suffer.
Like black hair is using the same curse to make it impossible for them to be together