To understand the workings of American politics, you have to understand this fundamental law: Conservatives think liberals are stupid. Liberals think conservatives are evil. — Charles Krauthammer
Even liberals who've accomplished a lot in their lives and have high IQs often say things on a regular basis that are stunningly, profoundly stupid and at odds with the way the world works. Modern liberalism has become so bereft of common sense and instinctually suicidal that America can only survive over the long haul by thwarting the liberal agenda. In fact, liberalism has become such a toxic and poisonous philosophy that most liberals wouldn't behave differently if their goal were to deliberately destroy the country. So, how does liberalism cause well-meaning, intelligent liberals to get this way? Well, it starts with...
1) Liberalism creates a feedback loop. It is usually impossible for a non-liberal to change a liberal's mind about political issues because liberalism works like so: only liberals are credible sources of information. How do you know someone's liberal? He espouses liberal doctrine. So, no matter how plausible what you say may be, it will be ignored if you're not a liberal and if you are a liberal, of course, you probably agree with liberal views. This sort of close-mindedness makes liberals nearly impervious to any information that might undermine their beliefs.
2) Liberals sources of information are ever present. Conservatives are regularly exposed to the liberal viewpoint whether they want to be or not. That's not necessarily so for liberals. Imagine the average day for liberals. They get up and read their local newspaper. It has a liberal viewpoint. They take their kids to school, where the teachers are liberal. Then they go to work, listen to NPR which has a liberal viewpoint on the way home, and then turn on the nightly news which also skews leftward. From there, they turn on TV and watch shows created by liberals that lean to the left, if they have any political viewpoint at all. Unless liberals actively seek out conservative viewpoints, which is unlikely, the only conservative arguments they're probably going to hear are going to be through the heavily distorted, poorly translated, deeply skeptical lens of other liberals.