Societal norms in many philosophical schools has come to be seen as the end-all determinant of rational life.
This attitude is displayed by Richard Rorty when he says "truth is simply a compliment paid to sentences seen to be paying their way."
Whether something is true, false, good, evil, rational, irrational, etc. is determined by the normative use of those notions in the society which you inhabit.
At the same time, a basic function in rational behavior is justifying your position on things with adequate reasons.
If we apply this call for justification to the before-mentioned position which sees societal norms as the ultimate explanation of everything rational, does it hold up?
That position, shown in the likes of Rorty among others, holds that what counts as a reason for something or not is a matter of the norms in a given society.
And if there are differing societal norms, in light of our sense of a need for justification, what lends support to the position that the ultimate determinant of rationality is societal norms if in another society our normative understanding would contend otherwise?
To Richard Rorty's quote we might say "Sure, let's grant that truth is merely a compliment generated by a society. I live in a society where your suggestion doesn't receive that compliment. What now?"