This post was heavily influenced by, and much of it blatantly stolen from, this post by Ace of Spades.
One can only discuss politics when conditions are exactly right. Both parties must be capable of distinguishing between opinion and self. Both parties must have a common understanding of the facts underlying an issue. These conditions almost never exist.
If a rightwinger's opinion turns out to have been wrong, the sane on the right simply change their opinion. To them, having held a wrong opinion doesn't mean they are bad or stupid people. Their existence is not justified by their political views.
The left - including the left which is on TV, or writing columns and books, or holding political office - are generally lightly educated persons. They consider themselves, by virtue of having memorized leftist dogma, to be intellectual superiors. Given their status as intellectual superiors, they feel a duty to instruct their inferiors.
Leftists claim superiority despite having rarely done intellectually rigorous work. They often haven't earned advanced degrees, nor undergraduate degrees, nor even - one suspects, in the cases of certain Hollywood actors - high school diplomas.
A leftist needn't be an actual scientist, an actual physicist, or an actual engineer. If said leftist memorizes enough dogma, he or she is automatically a gifted intellectual. Memorizing dogma = earning an equivalency degree. Memorizing dogma is a shortcut to all the self-esteem benefits, without doing all the actual, yucky work. Memorizing dogma equates to being a gifted intellectual. It justifies your superior, enlightened existence. You may begin to take yourself very, very seriously.
I'm not on the left. My existence is not defined or justified by any political opinions I hold. I'm willing to be wrong about my political opinions, then change them.
Persons of the left take their intellectual status very, very seriously. It's who they are. It's how they define themselves. It's the core of their self-image. Their special status is all dependent on their memorized dogma being true.
This is the spot where leftists are brittle: they don't actually examine the dogma before memorizing and proselytizing it. They take it on faith. They join the club. They figure: if such large numbers of cool and smart people already believe it, well, it must be true. Mustn't it? If cool people believe the dogma, and if the dogma feels right, and if it's kind of difficult to examine the underpinnings of the dogma with intellectual rigor ... well, then, why worry? I'm in! No time to waste! I can't wait to be a gifted intellectual!
Soon enough, they've built a sort of living monument to their own genius, and are thus locked into a condition in which they cannot examine the dogma - for it is the very basis of their gifted intellectual status. If any part of the dogma which is accepted by all the cool people is untrue, then many parts of the dogma which is accepted by all the cool people might be untrue. All parts of the dogma which is accepted by all the cool people might be untrue. Scary.
These possibilities must not be considered, for upon the dogma rests the self-image. If your persona is based upon leftist dogma, you cannot allow any portion of the dogma to be challenged. It is too dangerous. It could be like a tiny leak in a dam. It could lead to a destructive flood. Your larger group of beliefs could become devastated. You could discover you are just a regular schlub of a person, instead of an enlightened, gifted intellectual. This is too horrible to consider. Your years as an intellectual would be exposed as a pretense, and a lie.
And that is the nub of the problem of political discussion. To discuss whether a leftist belief is untrue is to attack the leftist's status. He or she will feel personally attacked. You are not simply discussing an idea, you are challenging their very persona - their very self image. You are attacking them. They feel attacked.
They cannot allow such an attack to go on. Their psyche cannot bear the possibility - however remote - that such an attack might succeed. Therefore, they strangle the attack in it's infancy. They refuse to acknowledge facts which might prove to be in opposition to their beliefs.
Remember, their beliefs are unexamined. Their beliefs are based upon faith in large numbers of cool people who endorse the dogma, and upon faith in feelings and instincts. At some level, the dogma goes clunk for them. However, always remember that clunk is emotional. Our intellectual overlords know they haven't examined the facts underlying the dogma. This scares them - as it should. In an attempted discussion, not only will they refuse to acknowledge any contrary facts to you, they will refuse to acknowledge any contrary facts to themselves. To do so would threaten everything they've built themselves up to be.
When confronted by contrary facts, Leftists are thus thrust into a state of cognitive dissonance. In their carefully constructed reality, they are our intellectual superiors. Any facts which threaten that status are invisible to them. They cannot see them. They do not see them. They cannot acknowledge them. They do not acknowledge them. To do so would feel almost life-threatening. They personify denial.
You might believe you are discussing the facts of a situation with them. They feel you are attacking the very core of their being. Your perceived "facts", if they threaten leftist dogma, are very, very personal threats to them. They will refuse to acknowledge the existence of your facts. Your facts do not exist. Their perception and reaction have nothing to do with discussion of truth, and everything to do with protecting their psyche, and with protecting their carefully constructed intellectual facade.
Which is why one can only discuss politics when conditions are exactly right. Both parties must be capable of distinguishing between opinion and self. Both parties must have a common understanding of the facts underlying an issue. These conditions almost never exist.