In his dialog with the famous psychologist Paul Ekman, the Dalai Lama explained why it is important to keep the mind cool.
With the calmness of mind our intelligence can function more appropriately. The intelligence is the instrument to know what is good, what is bad, and what are the consequences—of the temporary and long-term. (Emotional Awareness, p. 170)
These consequences are both ethical and personal. Failing to grasp the consequences of your actions, you will fail to optimize both the universal and your own well-being. The solution is mindcoolness.
Calmness of mind. Then your intelligence can be used effectively. Intelligence basically has the capacity to know reality. Most of the distracting emotions are very much connected with being uninformed or misinformed. (Emotional Awareness, p. 172)
Ekman had advised His Holiness not to use the word “ignorant” because it would make him sound arrogant and judgmental. Yet what exactly does it mean for emotions to be “connected with being uninformed or misinformed”? The Dalai Lama continues:
If someone, their state of mind is full of anger—or fear, or jealousy, or lust—during that period, their intelligence cannot function properly. During those periods, judgment often goes wrong. […] Why? The lack of calmness of your mind. Your mental or natural capacity then no longer functions properly. Because of raw emotion, which is very much mixed with distorted reality and absence of knowledge. (Emotional Awareness, p. 173)
In short, emotions distort reality, distort knowledge, distort judgment. By heating up the mind, they impair its functioning. This is not a new insight, of course, but an important one to contemplate time after time. If you think you know it all and need not hear it again, think again. Emotions tarnish even the greatest, most rational minds.
Now, you could quote the famous case of Phineas Gauge to argue that all human decisions, including rational ones, require emotion. I agree, but if you have been reading this blog for a while, you know how I have solved that paradox—namely, with the rational emotion of true pride.