Maybe I didn't quite word myself clearly - I am not saying "all people are always exercising only reason", but I am saying that "no person likes to believe that they are crazy" - and the common definition of sanity in this day and age is behaving "rationally". This is why you often see people coming up with justifications/excuses for their irrational behavior rather than accepting the fact that they act irrationally. I think that the reason that we want to believe in a rational mind, is because it provides a framework within which we can predict other people's reactions and responses, and therefore allows us a consensus in which we can operate as a society. I would think that the fact that your mind and cognition has flaws that are invisible to yourself is an uncomfortable thought for most people, and furthermore, most people are not introspective to that degree of self-awareness for the question to come up in the first place. This is why the evidence of these flaws is surprising and distressing each time we encounter it, and why these flaws are so dangerous, because when they are exploited, our thinking can be modified without awareness of being manipulated. If you're interested in rationality and cognition, take a gander at lesswrong.com - it has an extensive library of discussion and writing on the subject. I'm starting to work through it myself now, I'd love to have some discussion about the concepts.
My initial research indicates that this is probably the case, if you substitute "researchers in particular disciplines" for "we." Apparently, this began in earnest shortly after WWII, when economists, strategists, AI researchers, and geneticists all adopted a re-interpretation of Enlightenment-era views of how people behaved in society, based on a highly-selective and out-of-context reading of their work, in order to make their own work more easily quantifiable. Being experts, their work informed public policy in the late 20th century, and so their ideas filtered out into society at large. I've posted a couple of documentary essays I've found while researching this, if you're interested. One is on game theory, and another is on the computer metaphor.I think that the reason that we want to believe in a rational mind, is because it provides a framework within which we can predict other people's reactions and responses, and therefore allows us a consensus in which we can operate as a society.