Do you remember when your friend made a bad choice, a wrong decision like walking out of a marriage, or quitting a job over a petty disagreement, or allowing a disagreement to break a long-time friendship?  And you said, "That person isn't thinking straight!" 

Straight thinking cuts to the core of the issue and puts the whole in perspective.  It gives you a grasp of the possible consequences, something we rarely have when our decisions are cluttered with anger and bias.