Consider the case of a man who had a good job and an IQ ranging in the top 3 percent, until he suffered some damage near his orbitofrontal cortex (OFC), the bridge linking our emotional amygdala and the rest of our brain. This stunted his ability to emote but kept his intelligence the same, but instead of turning into an unstoppable logic machine, he found himself struggling with even the simplest everyday problems. What to have for lunch, what kind of pen to use, where to park his car -- all these simple decisions turned into "what should I say in my inaugural speech?" level dilemmas. He's not a unique case, either: Patients with OFC damage often become completely rational but can't make decisions -- because they don't have emotions to lead them the right way.
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"All right, penis, looks like you're taking the lead from now on."
The mind likes to use emotions to lead us to the correct decision by sprinkling the best and worst options with visceral emotion: The awful options (say, placing your hand on a hot stove for support) comes laced with fear and negative emotion, while the decisions your brain deems best come wrapped in a tiny cloud of happy. It's basically a people version of that thing with Pavlov and the dogs. You don't have to expend brain horsepower when thinking through the "do we pet the snarling dog or not" decision -- your knee-jerk fear makes that choice for you.