Why do you think they changed the book to make the movie? Use specific examples of when the added or took away parts that were included in the book. For example:
The Hobbit Movie added the Pale Orc to the storyline. This wasn't in the section of the book that they covered in the movie.
I believe they did this to add inner conflict and motivation to Thorin to help give him character and a background of why he is more serious and strict than the other dwarves. This also adds a good and interesting subplot to the film and you wonder throughout the movie, what will go down between Thorin and the Pale Orc.