Zuckerberg once openly stated that the core reason for refusing Yahoo's $1 billion acquisition in 2006 was very simple.

Everyone around him kept urging him to accept, after all, this amount of money far exceeded the company's actual size at the time, but he genuinely believed in what he was doing.
He asked himself at the time: if I sell the company, what would I do? The answer was that he would most likely start another similar one—and he already loved the current company.