Let's get real here: Every entrepreneur is not starting a company "to make the world a better place." And admitting your real motivation, selfish or selfless, is crucial to your company's success.

Many founders have in fact changed the world for better, and many have strengthened communities, created jobs, and built incredible products. But the real reasons many entrepreneurs start their own companies are different from the fluffy, smiles-and-rainbows missions Mike Judge pokes fun at in HBO's "Silicon Valley."

It all comes down to ego. That's according to Derek Lidow, a professor of entrepreneurship at Princeton University and founder of data analysis firm iSuppli Corporation, writing in Harvard Business Review.

"When I meet entrepreneurs I always ask them why they started their companies and they almost always say something like 'because I had a great idea the world needed.' But when you peel back the layers you discover far different motives--motives they don't want to acknowledge because they're directly related to primal desires and fears," Lidow writes. "Yet the well-being of their businesses depends on their understanding those real motives."

You know what you're passionate about by thinking for a few minutes. But pinpointing your motives driving that passion takes some digging and soul searching.

"An entrepreneur's true motivation can thwart their success if it is left unstated or conflicts with the interests of the business," Lidow writes. "Because changing your deepest motivations is almost impossible, you must find a way to put them to work for you by aligning your motivation with your new venture."

Saving the world may be noble, but it's often not what drives entrepreneurs to start companies. But if you do acknowledge your selfish motivations, Lidow says your employees, partners, and investors will have more confidence in your success and have a clear understanding of why you're taking on the challenge.

Check out Lidow's three examples of selfish--but true--motivations that drive entrepreneurs.

To do what you enjoy.

Lidow says a friend of his, a talented coder named Abe, started a company because he said he had a "great idea the world needed." Abe's talent and business model attracted investors. But all the money, new employees, and tons of customers created work he hated. After a while, the investors fired him.

"Only after he was fired did Abe's soul-searching lead him to realize that his real motivation was to work only on projects he enjoyed," Lidow writes. "Only after the fact did he realize that by bringing in outside investors his life became exactly the opposite of what he wanted, as he was forced to work only on what everyone else wanted him to."

To be the top dog.

Another entrepreneur named Bruce, who has a "brilliant mind" but a difficult personality, started four companies that eventually failed. Bruce thought he was motivated by the potential to make tons of money; but after the fourth startup dissolved, he spoke with an analyst who helped him realize it was not wealth he was after but rather a need to "be top dog" at anything he put his mind to.

"He needed to start a business in an area where his skills were world-class rather than simply targeting the latest lucrative industry, playing to his strengths and fulfilling his desire to compete aggressively," Lidow writes. "Understanding his need to be top-dog also helped Bruce realize that he wanted to get all the help and VC money he could to help him grow his enterprise as fast as possible."

Today, Bruce's fifth startup now has a billion-dollar valuation--and it doesn't bother him that the company's investors will be taking a big chunk of the loot.

The need to be needed.

Lidow writes about his process of understanding his own selfish motives while working with an executive coach. He found he had a "need to be needed," which made him a bad team player if no one needed his help.

"Once I realized my core motivation it changed my life, driving me to be an entrepreneur in an area where my expertise and advice were highly regarded," he writes. "At my startup, iSuppli, I created an organization structure that revolved around weekly status meetings and quarterly business reviews that allowed me to feel clued in and integral to every part of the business, without micromanaging or holding back our growth."

The company was sold in 2010, a success that Lidow attributes to this realization.