Young Adult Literature, Dystopian Literature, Adventure
Young Adult Literature
Not only is Tris a young adult, but also she's dealing with some classic issues of young adulthood: choosing a life path, separating from her family, realizing that her parents aren't complete morons, a bit of smooching.
Dystopian Literature
The future in Divergent isn't as bad as it is in some books, like 1984 (where it's impossible not to be miserable). But it's pretty bad. Chicago is partly in ruins and who knows what the rest of the country is like. On top of that, the whole social situation isn't so great. Do you really want to live in a world where you have to choose one faction and the factions stay so rigorously separate?
Adventure
But the book doesn't feature Tris moping around about how she's only a young adult and lives in a dystopia. No, Tris is always doing something adventurous: she's fighting in the arena, she's going on quests to find and save her family, and she's fighting the evil Erudite plans. So at least she manages to keep busy.