Just like the Starks, they want to be together at home. The Night King in wants to take back his old home/land from the Children of the Forest have taken and must to get some variation.
The Children of the Forest is the one to start this problem in experiment this human. It assumed that the Children in captured an incent human and in buried dragonglass in guy’s chest. I am sure the Children can't control this new blue eyed-human. He got trauma, changed his is emotion state, and evolved a negative character that got more power-magic than the Children.
Do you think the Old Gods were not involve – incent – this event. The Children roped the Night King on side the weirwood tree. Do you think the Old Gods have told the Children, “hey lil green-girls, you shouldn’t do this, it’s a bad idea…he might be a little pissed?” Or do you think, the Old Gods have slept hangover a bender this whole time. I don’t think so.
What about this old 3-Eyed Raven – before the Bran. Does his timeline follow the Children experiment? He should fix this…
My own theory, I believe the Night King came from the Starks family and lived at Winterfalls. He wants to take back the North and crack inside the crypt and show everyone these secrets that happen. I always thought they had some dark secrets about Starks family. Specific why they need a Stark person to guardian of Winterfalls.
Plus, this history of why Bran the Builder in create the Wall, Winterfalls, and Citadel…. They could have killed all these the Night King, White Walkers, and Wights over thousands of years ago when they could have killed them easy-weak enough. Instead they pushed him go top North and build the Wall.
Know the Night King have built up power/magic and create an army that now is come back… What do you think?