Here is a nice video of one of the world's great venture capitalists, John Doerr, talking about the different motivations for entrepreneurship. Drawing on Randy Komisar's The Monk and the Riddle, John contrasts the "mercenary" to the "missionary" approach to launching a new business or initiative.
Randy's book was one of the things that gave me the courage to leave the World Bank and co-found GlobalGiving, and Randy has been a steadying influence all these years. I have benefited greatly from his optimism tempered with realism. In his book, he notes: "Success, even on your own terms, entails sacrifice and periods of very hard work." Amen to that.
(Thanks to Paul Kedrosky at Growthology for the tip.)