Last week, Tesla CEO Elon Musk, explained why he works 120-hour weeks in an interview with tech journalist Kara Swisher. Swisher asked, "Why do you push yourself that hard?" Musk responded: "Well, the other option would have been, Tesla dies."

Tesla cannot die, Musk said, because it's important for the future of sustainable energy. "The fundamental purpose, the fundamental good that Tesla provides is accelerating the advent of sustainable transport and energy production," Musk said. "It's very important for the future of the world."

This isn't the first time that Musk has referred to an overarching sense of purpose as the principle driver behind his ferocious work ethic. Earlier this year Musk said, "Life cannot just be about solving one miserable thing after another. That can't be the only thing. There need to be things that inspire you, that make you glad to wake up in the morning and be part of humanity."

Management book author Marcus Buckingham once conducted a study of thousands of peak performers. He arrived at what he considers the single best definition of leadership: "Great leaders rally people to a better future," he writes in The One Thing You Need to Know.

According to Buckingham, a leader carries a vivid image in his or her head of what a future could be. "As a leader, you are never satisfied with the present, because in your head you can see a better future, and the friction between 'what is' and 'what could be' burns you, stirs you up, propels you forward. This is leadership."

Elon Musk's vision burns him, stirs him and propels him forward. Steve Jobs was driven by a similar purpose.

The one reason Steve Jobs returned to Apple

In Walter Isaacson's book on Steve Jobs, he wrote about the day Jobs returned to Apple as an advisor in 1996. Jobs was president of Pixar at the time, and went to the company's headquarters to break the news to the Pixar team. Although he would still be involved in Pixar, he wanted them to know that Apple would take a lot of his time. Asked why he wanted to try to save Apple, which was facing severe financial problems, Jobs said: "The only reason I want to do this is that the world will be a better place with Apple in it."

Inspiration, by definition, means to be "imbued with the spirit," an almost supernatural drive to accomplish something great. I've rarely met a successful entrepreneur who is driven by money alone. They enjoy the money and the opportunities wealth provides, but they show up to work because they're driven by something bigger. They have to be.

Starting a company is hard. Without passion for a greater purpose, it's nearly impossible to stay positive and keep going when the inevitable hurdles come along.

In the interview with Swisher, Musk vividly explained the "excruciating effort" it takes to start a company from scratch. "Making a car company successful is monumentally difficult," said Musk. "There have been many attempts to create a car company and they have all failed... So, as a startup, a car company, it is far more difficult to be successful than if you're an established, entrenched brand. It is absurd that Tesla is alive. Absurd. Absurd!"

It seems clear to me that Musk isn't driven by making money or even by making a better car. His core purpose appears to be doing something to save the planet from environmental catastrophe. Steve Jobs wasn't driven by money or even to make a better computer. He was inspired to create tools to help people unleash their personal creativity.

Before you start a business, make sure you're inspired by a purpose that will keep inspire you to jump out of bed every morning, work weekends, and stay late at the office. It's the only way you'll be able to really succeed.