I am dubious of the proposition that humans are all that rational. Here is a longish list of reasons for such doubt.
List of cognitive biases – Wikipedia, the free encyclopedia.
Here are some of my favorites
Availability cascade A self-reinforcing process in which a collective belief gains more and more plausibility through its increasing repetition in public discourse (or “repeat something long enough and it will become true”)
Bias blind spot The tendency to see oneself as less biased than other people, or to be able to identify more cognitive biases in others than in oneself.
Confirmation bias The tendency to search for, interpret, focus on and remember information in a way that confirms one’s preconceptions.
Conservatism (Bayesian) The tendency to revise one’s belief insufficiently when presented with new evidence
Endowment effect The fact that people often demand much more to give up an object than they would be willing to pay to acquire it.
IKEA effect The tendency for people to place a disproportionately high value on objects that they partially assembled themselves, such as furniture from IKEA, regardless of the quality of the end result.
Information bias The tendency to seek information even when it cannot affect action.
Dunning–Kruger effect An effect in which incompetent people fail to realise they are incompetent because they lack the skill to distinguish between competence and incompetence. Actual competence may weaken self-confidence, as competent individuals may falsely assume that others have an equivalent understanding.