It’s very easy to see yourself as a rational person. You have a bunch of information and experience, and based on that data, you make decisions. You are the only person who knows precisely what you know, and therefore you are the only one making decisions based on that knowledge.
For the same reason, it is very easy to believe other people are irrational. Your analysis of them being rational or not is based on the knowledge you hold, but they base their decisions on their understanding and information. Therefore, it is often the case that people believe that they are rational and that others are irrational, but really, we are all just as rational or irrational.