Write or print this on a BIG piece of paper and stick it on a wall where it's in-your-face.
Just because people use reason, that don't mean they're rational.
This is what one might call the 'Objectivist Fallacy' (that is, the Rand-version of 'Objectivism', as opposed to the metaphysical kind): the notion that what has become a tool for survival, developed and refined through the process of evolution, is actually somehow 'fundamental' to how we should understand ourselves and our essence.
The evidence to support the Objectivist Fallacy is so close to non-existent that it's almost on par with the evidence to support the existence of God. On the other hand, there is evidence, as for example popularized by Dan Ariely, that irrationality is indeed our basic mode of functioning. The joke—on Rand-ian Objectivists; and that's only one of the jokes, because they unwittingly deliver others quite of their own accord—is, of course, that we can use reason, that powerful tool, to analyze irrational behavior; to the extent of actually being able to predict its occurrence and actions.
The whole thing came up, BTW, because a friend of mine simply appeared at a loss to explain what he considered the irrational behavior of tenants who rent a part of his rural property. Why would they act such and such, if this made no sense whatsoever?
And surely the very assumption that other people share one's own sense of what is rational and sensible is in itself entirely irrational. The results of the use of the tool of reason is entirely dependent on pre-existing assumptions—like my friend's mistaken one! That's the danger with tools. To someone skilled in the use of a hammer every problem looks like a nail. To someone skilled in the use of reason, every problem looks like it's just waiting there for a rational solution. Which is may well be and in some cases it will be subject to rational resolution; but if said problem relates to matters of psychology, individual and social, the only workable assumption from which to proceed is that other people use reason but are not actually motivated in their action by it.
That it should be different may be closely related to this snippet of information:
...when we use a tool—even for just a few minutes—it changes the way our brain represents the size of our body. In other words, the tool becomes a part of what is known in psychology as our body schema...
I would like to submit that the processes that enable the Objectivist Fallacy to exist, are related to processes not dissimilar to this. It's not exactly the same thing, of course—after all, the 'tool of reason' is internal, if you will, and usually implicit, so we don't actually think about it as a tool. But that may be exactly the reason why the confusion arises to easily, persistently and ubiquitously.