In the comment thread to my post on presuming good faith, commenter Bluquark writes:

I think many cases of apparent bad faith are mainly self-deceit. People very commonly ignore or distort evidence that goes against their preconceptions. It can be difficult for an outsider to tell the difference between this, and a more conventional lie designed to deceive only others.

  I would put this point differently: Levels of self-awareness vary, and people often make arguments instinctively. They see themselves as being on a side, and they have a general sense that their side is right and the other guys are wrong. As a result, people often feel comfortable grabbing an argument that comes along even if they haven't thought it through carefully. Of course, there's a big difference between making an argument you should know is weak and making an argument that you actually don't believe.