The crux of the matter is that "reason" is not absolute. People rarely do things that they (at the moment of committing the act) believe to be unreasonable - generally, every action taken seems quite reasonable "at the moment". It is only retrospectively that they realize that their past actions/thoughts were unreasonable.

There is a very nice discussion on the relativity of rationality in the book "Instincts of the herd in peace and war" by Trotter. I copy a relevant paragraph below:

The religious man accuses the atheist of being shallow and irrational, and is met by a similar reply ; to the Conservative, the amazing thing about the Liberal is his incapacity to see reason and accept the only possible solution of public problems. Examination reveals the fact that the differences are not due to the commission of the mere mechanical fallacies of logic, since these are easily avoided, even by the politician, and since there is no reason to suppose that one party in such controversies is less logical than the other. The difference is due rather to the fundamental assumptions of the antagonists being hostile, and these assumptions are derived from herd suggestion ; to the Liberal, certain basal conceptions have acquired the quality of instinctive truth, have become " a priori syntheses," because of the accumulated suggestions to which he has been exposed, and a similar explanation applies to the atheist, the Christian, and the Conservative. Each, it is important to remember, finds in consequence the rationality of his position flawless, and is quite incapable of detecting in it the fallacies which are obvious to his opponent, to whom that particular series of assumptions has not been rendered acceptable by herd suggestion.