Mark Zuckerberg and friend in Noe Valley Tim Williamson Poor Larry Page and Mark Zuckerberg. They're worth billions of dollars, and that makes life hard.

Actually, it kind of does, and Y Combinator co-founder Paul Graham explains why. Graham's startup accelerator program yielded a few billionaire founders of its own, including Dropbox's Drew Houston and the founders of Airbnb.

Graham points out that Google and Facebook run Page and Zuckerberg as much as they run their companies. They can never enjoy regular activities or do things spontaneously the way the rest of us can.

"Mark Zuckerberg will never get to bum around a foreign country," Graham writes. "He can do other things most people can't, like charter jets to fly him to foreign countries. But success has taken a lot of the serendipity out of his life."

What adds to the stress: billionaires can never publicly complain about how tough their money-filled lives sometimes are, because everyone else will freak out.

In a Stanford talk about how tough it is to be an entrepreneur, Graham says how all-consuming running a startup — even once it becomes a multi-billion-dollar company, can be. Here's an excerpt:

Larry Page may seem to have an enviable life, but there are aspects of it that are unenviable. Basically at 25 he started running as fast as he could and it must seem to him that he hasn't stopped to catch his breath since. Every day new shit happens in the Google empire that only the CEO can deal with, and he, as CEO, has to deal with it.

If he goes on vacation for even a week, a whole week's backlog of shit accumulates. And he has to bear this uncomplainingly, partly because as the company's daddy he can never show fear or weakness, and partly because billionaires get less than zero sympathy if they talk about having difficult lives. Which has the strange side effect that the difficulty of being a successful startup founder is concealed from almost everyone except those who've done it.

...Mark Zuckerberg will never get to bum around a foreign country. He can do other things most people can't, like charter jets to fly him to foreign countries. But success has taken a lot of the serendipity out of his life. Facebook is running him as much as he's running Facebook. And while it can be very cool to be in the grip of a project you consider your life's work, there are advantages to serendipity too, especially early in life. Among other things it gives you more options to choose your life's work from.