Most conspiracy theories don't make sense nor withstand any scrutiny. They usually involve operations so immense that it's basically impossible for them to be kept secret, and all the proof given by conspiracy theorists usually have a very simple explanation (usually much simpler than the explanation given by the theorists).
Yet conspiracy theories are very popular and appealing. Even when they don't make sense and there's just no proof, many people still believe them. Why?
One big reason for this is that some conspiracy theorists are clever. They use psychology to make their theories sound more plausible. They appeal to certain psychological phenomena which make people to tend to believe them. However, these psychological tricks are nothing more than logical fallacies. They are simply so well disguised that many people can't see them for what they are.
Here are some typical logical fallacies used by conspiracy theorists: