Movie plots are usually different from the books or other source material they are made from. First off, they have to be made shorter; after all, few bestsellers can be read in two hours or less. But the story is also sometimes changed to give it a happy ending, which these days is considered crucial for box office success. Neither Gone With The Wind nor Casablanca had a happy ending, but both films did okay. Those movies weren't screened for focus groups before release. If a focus group of typical movie viewers doesn't like something about a movie, it can be changed by deleting scenes or even filming reshoots to change the story.
Adding a happy ending isn't the only reason movies get changed. Sometimes scenes are deleted before they are even shot. Mental Floss looks at seven films you know and love and how and why they were completely changed from the original plan.