“That’s a pretty weak motivation IMHO (the Night King wants the Long Night to return)”
Maybe the three-eyed raven is lying, to manipulate people into fighting his enemy for him.
I’m not saying the Raven is lying, though that would be a twist wouldn’t it?...
I am simply saying its pretty 1 dimensional...
Perhaps the books have offered more in terms of the history of the world...
Obviously they were beaten back to the north (or retreated) there at some point in the past and the rest of the world built the wall to keep them there... They have lived there so long, they are all but forgotten by humanity, to a point where they are believed to be nothing but Myth and Legend... things of stories to scare young children.
Many winters have come and gone since they were driven or went north...dragons too are all but forgotten myths... So, why NOW... why this moment does the Night King return, or attempt to return to take the southern lands...Why not any time before? Etc etc...
Maybe the end game is everyone is dead and the Night King controls Westeros, and humanity is bound to Essos... Until (if ever) the Night King decides to cross the sea...
Doubt that, but who knows.