Fear of conflict and confusion of inconsistency
Most people don't like to disagree. At least not for more than a few minutes at the time. Hence, resolution has a tiny window of opportunity to be reached. Once that window closes, the person will loose the capability to accept a new position. Regardless of the arguments.
I don't mind resolving a difference of opinion over a longer period of time, so this is always a disappointing, occasionally infuriating, discovery of character.
People tie identity to opinions and the consistency of these. Inconsistency is frowned upon. Some times with good reason, mostly without. The inability to distinguish the willingness to accept a new point of view, positive "inconsistency", with the failure to make up your mind on a fixed set of information, negative inconsistency.
In face of the difficulties in seperating the two kinds of inconsistency, most people take the pessimistic route and brands all in the negative category.
This is primarily a problem of self-perception. We don't want others perceiving us as inconsistent. When ever we're approaching the border of inconsistancy, the fangs of aggression come out to vigorously defend the self-image.
The trade, however, is rarely worth it. If the arguments are clear and strong enough, the sense of inconsistency can't be shaken. And now on top of inconsistency, you add a layer of aggression and retraction. Combination: Jerk.