Books are several hundred pages long, you just can't fit everything into a movie. Fellowship of the Rings is a good example. They literally cut out hundreds of pages of story, they changed the story up, they changed characters around and eliminated at least a dozen of characters, and that was just the first book. Yes the movie was wonderful, it was just brilliant but it wasn't a complete telling, it was a stylized version. They tried to stay as true to the book as possible but it isn't the 100% story it's a edited version. Another one? try Dune, the movie or the mini-series, neither were 100% on the money, matter of fact both drifted pretty far off the books path. Both were good for their own reasons but neither matched the book.
Books and movies are two different mediums, sometimes movies based on books are actually better than the book was, but there will always be differences.