Jonah Lehrer explains how we rationalize:
Human reason has nothing to do with finding the truth, or locating the best alternative. Instead, it’s all about argumentation...So here’s my new metaphor for human reason: our rationality isn’t a scientist it’s a talk radio host. That voice in your head spewing out eloquent reasons to do this or do that doesn’t actually know what’s going on, and it’s not particularly adept at getting you nearer to reality. Instead, it only cares about finding reasons that sound good, even if the reasons are actually irrelevant or false.