In December, my wife, Elaine, and I attended a 50th-anniversary concert of the Rolling Stones at Brooklyn, New York's new Barclays Center. They're often referred to as the Strolling Bones these days, and with good reason. They're old. You can't help noticing how weathered their faces look. So it's all the more amazing to watch Mick Jagger, at 69, strutting around the stage just like he did half a century ago. I couldn't take my eyes off him. He and I are about the same age. "Why is he doing this?" I asked myself. "He doesn't need the money. What is it?"

Then it hit me. I dropped down into my seat and started to laugh. "Why are you laughing?" Elaine shouted over the din. "I just figured out what's been driving me all these years," I shouted back.

As I told her later, I'd had an epiphany. I'd suddenly understood the meaning of the word passion. Mick Jagger, I realized, keeps doing what he does for the same reason that I keep doing what I do: because he has to. Performing defines him. He has to do it in order to feel alive--which is also why I start and build businesses.

I'd never appreciated that before. Sure, I've long known how much I enjoy watching a business rise up from nothing. But I didn't take in that there was something deeper going on, and it wasn't about the business. It was about me.

That was a revelation. Like most people, I think, I'd tended to view passion as simply an extreme form of enthusiasm. For that reason, I'd always rejected the idea that some people were born to be entrepreneurs. After all, the knowledge and skills required to build a business are teachable, and anyone can be enthusiastic.

Watching Jagger made me realize that enthusiasm and passion are different. Enthusiasm has to do with the way you feel about something that happens outside you. Passion comes from within. You don't need passion to be successful, although I do think that enthusiasm is essential. If you aren't enthusiastic about building a business, you won't have fun doing it, and you won't have the resilience required to succeed. But I know many successful business owners whom I'd never describe as being passionate about what they do.

Then again, if such a passion is inside you, you'll be better off accepting it--because, like it or not, it's going to play a big role in your life. And it's always a good idea to know what's driving you.