What evidence is there that Iran would be this irrational actor that you fear?
The problem isn't that they may be irrational, the problem is that they ARE rational. Rational actors seek power to maximize their security hence nuclear power. More power is ok for me in a stable liberal government. That's not ok in a country run by imams in a very volatile part of the world where we hold important regional interests. If we can prevent it, we should. We are behaving rationally, it would be irrational of us to allow them to increase their relative power politically and militarily in very volatile part of the world where we hold important interests.