This Game of Thrones Season 7 theory takes a deeper look in to what the Night King and the White Walkers are after! He left us an epic message that traces back to the Land of Always Winter!
What did the Night King lose when the COTF changed him? Family? Likely a loved one. I think he wants the power of the 3 eyed raven. He couldn't find him until Bran revealed their location. Brynden passed the power onto Bran before the Night King could take it from him. Ironically Bran and the Night King's goals may ultimately align with each other, both may want to alter the past in order to prevent a terrible future. But what would the cost of that action be?