Why is it that when you are in an argument with someone they can't just stick to the subject at hand and make a big mess of it?

Arguments could be over and done with if the other person just stuck to the facts and didn't start accusing you of things that weren't true, putting you down, etc., until the two of you aren't even speaking to each other anymore. Have you ever had that happen to you?
Why is it that when you are in an argument with someone they can't just stick to the subject at hand and make a big mess of it?
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