Why is the movie never as good as the book?

It seems like every time I read a great book and see the movie afterward, it is always a let down. I recently read the book "My Sister's Keeper" by Jodi Picoult. What a great book! It was a tragic yet captivating story, very emotionally stirring. It can be very cleansing to have a good cry with tears gushing like Old Faithful, and this book delivers! The tears were streaming steadily throughout the book, but it was the ending that got me. I was a soppy mess as I finished that book! But it was such a great ending, in my opinion. THEN I saw the movie.....and they completely changed the ending! Why did they do that? It was such a good ending in the book, I don't see why it needed to be changed. Not only do movie producers change the wonderful stories around when they convert them into movies, but it seems they are always horribly mis-casted. In the case of "My Sister's Keeper", I'd have to give it a B+ on casting. I've seen worse mis-casted characters in other movies made from books (don't even get me started on "Twilight")! I suppose our imaginations and real life can never match up in perfect harmony. I think I will just stick to the books because they are always better and the characters will look the way I want in my head. You can't beat your own imagination!