1). Let's say, here we have a book. It's 365 pages in length, and surprisingly full of detailed action and character development. Normally, that would make it a great book. However, if we are to put everything on the big screen, we have a few difficulties. A normal screenplay is roughly 95 to 125 pages in length (read this link for more details). Therefore, we have to condense 365 pages into less than half its length.
A person able to accomplish this feat and still produce a great film to match the great book is a rare gift. Hollywood should worship the ground they walk upon.
2). When dealing with different mediums of stories, there WILL be changes. The audience changes, the way the story is told changes (i.e. first person to third person - a movie is always in a third person point of view). And the story is not being displayed because one person is doing this for their own enjoyment. The story is being displayed because a handful of people wanted to make a two-hour movie out of a 365 page book.
The addition of more people opens up a new door.
3). A movie is a collection of interpretations, not just one interpretation. We have our 365 page book again, only limited by the imaginations of its readers. Let's say, we have five people who read the book. The Director pictures the monster's horns curled inward, the Producer imagines the monster's horns curled outward, and the Screenwriter completely forgot the monster had horns. So what happens? They collaborate their views to create a final product. We are not getting the vision of a single person (maybe we are, depending on the director), we are getting a compliation of visions by multiple people.
So the next time you walk into a movie expecting to see its book counterpart: don't. You will always be disappointed.