Well, I'm not sure it's easy to pinpoint one thing that motivates Musk.
As I argue in the book, I'm convinced that Musk is what psychologists would consider to be a "profoundly gifted" person. This sounds like a fluffy, abstract term, but it's actually quite concrete in the scientific literature. The quick explanation is that Musk was a child with a very, very high IQ who also exhibited certain grandiose traits. Profoundly Gifted children often take troubles in the world quite seriously, dwell on them to no end and want to fix them. Sadly, many of these children are often overcome by the depth of their emotions and end up sort of paralyzed and unable to tap into their skills. Musk seems to have gone quite the other direction and become more and more capable over time and more and more ambitious.
Is this existential depression from his childhood? Yes, I believe so to an extent.
Musk read a ton of science fiction as a child and took the ideas that mankind should try and save the world and spread out through the universe very seriously. He was bullied at school and unhappy at home, which made him turn inward and think of ways to improve his life and the lot of others.
I think he deeply wants to help out mankind, while also proving that he is a special person who is deserving of love and adulation. (Caution: These views were expressed by a journalist with a degree in philosophy and a giant armchair.)
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