Yammer CEO David Sacks at Launch Festival 2013 Owen Thomas, Business Insider Seven months ago, one of the most successful entrepreneurs and angel investors in tech history, David Sacks, shocked everyone by taking the COO job at young, fast-growing startup Zenefits.

Sacks is a card-carrying member of the PayPal "mafia," who have gone on to massive success in Silicon Valley. He was the COO of PayPal from 1999 to 2002 until the company's sale to eBay, then founded two successful startups: Yammer, which Microsoft bought for $1.2 billion in 2012, and genealogy site Geni, which MyHeritage bought for $25 million in 2012.

He's also an an angel investor in companies like Twitter, Uber, SpaceX, Airbnb, Scribd, AngelList, Slack, Eventbrite, Eventbrite, SurveyMonkey, Lyft, and dozens of others.

He even quit the Valley for a while, moved to L.A., and produced the hit film "Thank You For Smoking."

He can do anything he wants. And yet, he chose to join a startup and not as CEO.

Why? Because he fell in love with the company, he says.

"Zenefits was honestly the most exciting company that I’ve seen that needed my help," he told Business Insider.

"I was talking to a few VC firms and being actively courted by them. Then I met with [founder] Parker [Conrad] and he told me his vision for this company. The clarity he had about what he was building, it was the clearest vision I’ve heard from a founder CEO. It got me very excited," he said.

Another reason: He's already done it all and isn't hung up on being the boss.

"It's less about what my title is and that sort of thing and more about just being able to work on a company. In addition to being a entrepreneur and having founded stuff, I've been active as an angel investor. So I've got experience being supportive of other people’s visions, I've always had the ability to do that," he said.

Zenefits co-founders Parker Conrad (left) and Laks Srini TechCrunch/Flickr So why not become a VC?

"I thought it would be more fun to go all in working on one company that I thought was most exciting, instead of spreading out efforts to me."

But there's one more reason that goes to the heart of it all: He thinks Zenefits will be the "next Google" in terms of its financial success. It's already one of the fastest growing startups in history.

"If I chose to be COO of some very big established company, a Google or something like that, you wouldn’t be asking why. That seems a lot less fun to me, like the heavy lifting was already done by other people. I think Zenefits can be one of those generational companies," he says.

With his track record on startups, a vote of confidence like that is akin to the Oracle of "The Matrix" telling Neo that he's "The One."

Sacks is also an investor in Zenefits, so like co-founders CEO Parker Conrad and CTO Laks Srini, he's not just an employee, but has a stake of unknown size.