Why do my parents fight so much?

In my experience, people will argue or fight when there is an expectation that has not been voiced or talked about between the two people. It is usually when one person has an expectation of the other person and when that expectation isn't met or understood by the other person, the disappointment starts in the first person and is usually not spoken about or addressed until it becomes a real argument. People argue over any number of reasons, but in my experience, it stems from an unknown expectation and disappointment that is left unchecked. In my experience there is usually a lot more that gets involved when two people have been fighting for awhile. It is also in my experience that the best way for the fighting to stop is for both people to be completely honest about their expectations of each other.