Books vs. Movies

The idea of turning books into movies has always been good in theory, however, in actuality it never works out. Why is that? Why do we feel this need for our characters to come to life? I guess the latter question is where most of the problems lie. When I read a book, and if the author does their job of sucking me into the story, I imagine the characters in my mind. They take on new quirks as they move fluidly throughout the story. No, the author may not have written about the anti-hero's sly smile as they spew their venomous words, but I picture it as part of the character just the same. So how can movie creators bring these beloved characters to the real world? 

The simple answer is they can't. Everyone has their own image and uniqueness they imagine for their fictional character and movies can't bring the fictional to reality in perfect form. No matter how close they get there will always be someone ticked off. While the perfect characters are hard to create surely they can perfect the plot. I mean how hard can it be? They basically have scene descriptions right in their hands! Apparently, it's very hard. Movie writers like to add in scenes that never happened or take out scenes, which they see as inconsequential. There have even been some extreme cases where the only similarity between movies and books are the title, character names, and a small thread of the original plot.
My personal source of anger with this circumstance is "Ella Enchanted". I first read this book in the fifth grade and attribute my love of reading to that book. While it isn't the greatest book ever written it is still one of my favorites, so I was ecstatic when the move was released. The movie, instead of being spectacular as I had imagined, was almost unrecognizable to my favorite book. The writers added musical scenes, scenes that never happened, and essentially created their own book/movie.So, my question is: Do you think we should stop putting so much pressure on books turned into movies, or do you feel that the anger is justifiable?