The New York Times wrote recently about "that gene that makes presidents want to be president".
The article includes a quote from Ted Sorensen, the former counselor to President John F. Kennedy: “You have to not only have a sense of confidence but a pretty big ego — you have to almost be a fanatic.” Other qualities mentioned include "ambition and drive" and "believing you have special gifts” and being attracted to the "elixir of adulation" and the "opportunity for immortality".
So where do all these qualities come from? What is it that makes a leader differ from a follower? Why doesn't everyone have a pretty big ego and self-confidence and ambition and drive? Why are those traits so rare? Can we all exhibit those traits, and simply choose not to? Why do most people feel comfortable as a follower?
Clearly, the brain is built by our genes. We have an innate brain module that senses crowds. Variations in our genes (from our neighbor's genes) cause us to react differently when this situation is detected. Some feel stress, and some feel excitement. You can't choose to have a big ego or not. It's part of who you are. You can't train someone to be motivated by power.
Depending on the gene variants (or flavors) you were born with, you choose (of seemingly free will!) to be one way or the other. From conception, your gene variants act as blueprints for unique proteins that construct your brain in various configurations. (After the development and configuration of your brain, those genes are then switched off). So once you're a few years old, your basic personality is established.
It may take many years until you experience a crowd for the first time, and realize how it excites you (or not). That's when leaders have their "aha!" moment. But most people are simply not born with "that gene [variant] that makes presidents want to be president". That is why leaders are so rare.