I've been re-reading Dennett's Kinds of Minds, and was struck by his suggestion that the key advantage in being able to think about one's desires, rather than simply following them, is that it enables you to be deceptive about them.
What he doesn't say, but supports the theory, is that it is easier to know what we want people to think we want, than it is to know what we really want. The implication is that our real desires are deeper than the reasons we defend them with. Our "reasons" are fundamentally for consumption by others.
This is a commonplace in politics, where the justifications for our political positions change more easily than the positions themselves. It is consistent, for instance, with Arnold Kling's recent piece on how the core beliefs of the US left are derived from theories that few still accept.
Not that the problem is confined to the left - that is just one example I read today. It will always be easier to spot that sort of thing in one's political opponents, but I think it is universal.
I don't mean to exaggerate and say that reason is entirely subservient to prejudice. But, if Dennett is right, then in order of priority, and of evolutionary chronology, unconscious intentions come first, then reasoned "approximations" to the unconscious intentions, and finally, feedback from conscious calculation to unconscious (or subconscious - I'm not sure of the technical distinction) intention or attitude. That feedback allows us to change our minds based on reason, but its status as a spin-off of other uses of reason explains why it is difficult for us to do so.