Jason Calacanis once asked Peter Thiel why Paypal produced so many great founders. What was it about Paypal? I loved Peter’s answer — Most people’s experiences with startups fall into one of two categories. Many work for startups that fail and learn that startups are impossible so they never try. Others work for startups like Google or Facebook and learn that startups are easy so they quit when it gets hard. Paypal was “just hard enough”. Early Paypal employees learned that startups are really hard, but it is still possible to succeed.

I don't know if that's true, but it sounds great. (source)