There is a birth defect that leads to people who have no sense of pain. They can handle hot frying pans without flinching. They can have their fingers viciously bit by animals without noticing. Awesome, right?
No, not awesome. The average life expectancy for these people is 17, because pain is actually a pretty good warning system against damage. People with the disorder have a nasty tendency to accidentally bite off their tongues, not notice that an object is too hot to touch until they have second (or worse, third) degree burns (and the first hint they get that it's too hot in the smell of burning flesh), and didn't learn lessons in childhood about dangerous things that get them killed, like that electrocuting yourself is a bad idea.
So when a woman recently lost her ability to feel fear to neurological disease, science was very interested. She is never afraid of anything. When she was taken hostage at gunpoint, she only became angry and upset and gave the gunman a thrashing. People who give her death threats are quite surprised when she responds with face-withering invective. And all this because something damaged her amygdala slightly.
If we reverse engineer what happened, it may come to pass in the future that we can turn off our own fear, something that militaries and police departments would surely pay huge amounts of money for. Same with the pain thing -- save bundles on anesthetic in any hospital. Just so long as you turn it back on again afterwards.