Homer Rice said “You can motivate by fear. And you can motivate by reward. But both of these methods are only temporary. The only lasting thing is self-motivation.”
I found this little gem at Casual Hardcore, a blog about various aspects of playing the online game World of Warcraft. This relates to so many things in life: everything, really! It is not dissimilar to this quote from John Holt or this one from Kurt Hahn.