We all like to believe we are acting rationally and that our opinions are based on sound reasoning and a thorough understanding of the historical record. But that doesn't turn out to be true. Instead, we make snap judgments about issues based on our values and then construct a set of evidence that supports our pre-drawn conclusion. I learned this from a liberal psychology professor, Jonathan Haidt, in this interview on Think.
[Originally posted as a comment on a Facebook post as preserved here to help me find the link again when I want it.]