Well, he didn't really want to be there in the first place, so it's not strange that he wants to go home.
Despite Tuuri just assuming, and not asking, I think one part may in fact be home sickness. I bet he loves night scouting, but he wants back home to *his* forest, maybe not because he misses it, but because he feels it's his job to take care of it.
But he might also have a bad feeling of what is up ahead, even if it's just for the reason that it's a city.
I do hope we get to know what "It" is. I expect "It" to be grandma's mistake, but what shape it has is of course anyones guess.