When someone is dying, you have the option to touch their forehead, thus dying instead—a mortality exchange. Parents will do this for a dying child; same with best friends, twins, spouses. The problems arise when someone is expected to do this for their partner but doesn’t want to; eventually they’ll mortality-exchange out of grudging obligation more than sincere commitment. When the revived partner discovers this, they'll re-touch the other's forehead, re-dying just to make the other feel bad. This often goes on for years, decades, centuries. Overall, it's usually best to let the dying die, and the deceased stay dead.