Ask HN: Why bother expanding?

I recently read that Andrew Mason of Groupon was forced (I presume he was forced) to step down of the company that he created. Same went for Steve Jobs first time around at Apple.

I don't understand why an individual, who pours his heart and soul into a startup/company and who sees that company flourish and make really great profits which affords that individual to have the life he or she wants to live, then bothers trying to expand? Why do they not stick at what they are good at, what made their business/product great? Is it greed? Or does a company have to expand to survive?

I run a small site. The reason I run it is because one day I would love nothing more than to work full time on it and be my own boss. If my small site made me enough money to live a fun life and to be able to do the things I would want to do then I can't imagine any situation where I would want to take my small site and expand it into an area that I didn't understand, feel comfortable with or introduce people into it that I did not know. What's the point?

I am not trying to be obtuse and I hope I am not being ignorant. I just don't understand why people would risk what they have (like Andrew Mason). So he earns an extra $x million, what could he do with that that he couldn't do with his existing fortune?