Film adaptations are what keeps Hollywood running, but it's a dangerous business to meddle with. Fans are touchy with their favorite books and are defensive over every detail, so it's impossible to please everyone.
Adapting to such a wildly different medium is never an easy business. It's absurd to put everything in a novel that runs for hundreds of pages into a two-hour movie. Whether it's trimming a plot, or changing something to fit the visual medium, these changes are necessary - but often determine the fate of the film.
It's not just fans who lament when filmmakers fail to adhere to the source material as closely as they'd like, but the creators. Many authors try to exert control over the movies bearing their names in the credits, for better or worse. It could be penning the script, reviewing the material, or advising the production. However, it's more common than not that their advice goes ignored, and the movies are all the poorer for it.
Some films based on books have been so atrocious that the author didn't simply refuse to endorse it, but launched a vendetta against what they saw as an insult to their work. There are cases where it's not enough just to condemn a film, but to ask moviegoers to avoid the adaptations entirely.