Why are people so irrational when it comes to politics? A possible evolutionary argument is that it was often a matter of life or death to be on the right side of a decision affecting the tribe. To be convincing in your arguments you have to be utterly convinced yourself. Our suspicion of foreigners is a case where our evolutionary heritage was probably important; tribes that were friendly to outsiders probably got killed by them. This is the answer I'd go for, but I'm also suspicious of this kind of evolutionary argument.
Why is free speech such a firmly established principle in western societies? Even people trying to undermine it usually pay lip service to the principle. What makes it different from other forms of freedom that are not generally accepted in western countries? It's easy for libertarians to get depressed about politics because nobody agrees with us :( But free speech should be enough to make us smile.