There’s a fine line that adaptations of novels have to walk. Leave too much in and you risk your movie becoming plodding and boring. take too much out and you risk dumbing down or losing a theme or moment or sub plot integral to that story. Nowhere is this more clear than with Young Adult novels. Because they are generally fairly easy to follow in the first place the smallest choice a director makes can have drastic consequences for the movie you are making.
Case in point, Divergent is a movie that I am sure is based on a good book that discusses and explores interesting themes, but the movie itself glosses over all of this to tell a pretty by the numbers story about a girl in a not-that-dystopian future.