As I understood it from the World of Warcraft story Arthas the Lich King and Jaina Proudmoore had an affair before Arthas turned out on his father and went to search for the Lich King’s armor after fiding frostmourne. The sword that he found hidden in a glacier was always directing him to his full armor because The Lich king wanted to be complete not just partially equipped so the sword directed Arthas to his full-body plate armor and then in that moment Arthas understood that he was the Lich King, not just some external force directed him it was himself.So before he found frostmourne he was in love with this beautiful mage named Jaina, we all know about her we can find her in World of Warcraft and Heroes of the Storm which is free to play. And the story is that they fell in love and promised to be forever always, but when the purging of Stratholme came it was time for Arthas to choose between cleansing the City and his love and the Silver Hand paladins wich Uther the Lightbringer was part of.
He neglected his love for Jaina which haunted him for times pass and that love just wouldn’t go away even when he was the Lich King he always remembered her and his love, not even the evil could stand against him and that love affair. They promised each other they would sit side by side and even get married I think.
But instead to choose a life of love and light Arthas decided that power was more important for him and for the greater good he would cleanse the Stratholme city of evil, for a good cause of course.
But that really wasn’t the case evidently because he could control frostmourne, the powerful sword wanted more, wanted Arthas to be complete and reign over all scourge in Northrend.
So the story is that no matter how in love with power you are, you would always remember your first love no matter how evil and thirsty for power you think you are.