When we don't know the reasons behind our choices, we confabulate.
In a now classic experiment, the psychologists Richard E Nisbett and Timothy Wilson at the University of Michigan laid out a range of items, such as pairs of stockings, and asked people to select one. Participants consistently preferred the items on their most right-hand side. But when they were asked to explain their choices, they did not mention the position of the items, and instead attributed their choices to the superior texture or colour of the chosen pair of stockings, even when the displayed pairs were all identical. People confabulated. Not knowing some of the factors that were determining their choices, they produced an explanation that was not based on evidence relevant to the factors determining their choices, but mentioned instead plausible reasons why the chosen item was better.