When one puts careful thought into why a book was adapted to a movie in a certain way, and then voices a complaint, receiving a response of "movies aren't books, you have to change things" really is kind of infuriating. Really? I had no idea. Try thinking about it and asking why a person thinks something before you shoot their idea down as irrelevant. You might be surprised.
Also, reacting to people who praise what was changed with "it wasn't that way in the book" can be equally annoying. Sometimes things really were better and the people praising it don't know or care, but sometimes they have put a lot of thought into it as well. Never assume, always question and listen first. You might learn something.
Of course this applies to most conversations, not just movie discussions :)