My rationality lesson: sometimes even rational people need to be irrational and just vent. Learning to tell venting from rational discourse was essential for me. The difference is not always obvious to either side, and an explicit clarification is usually in order. (I suppose that it should be agreed upon first that this question is OK.) It is counterproductive to argue with your partner at this stage.

Once s/he lets out the steam and is in a condition to think more or less clearly, (and maybe a few hugs and affirmations of mutual love and respect later) a reasonable discussion might begin. If a dialog turns into a fight, then more often then not at least one side (and usually both) did not get a chance to vent.

As usual, YMMV.