I get the best sense of this key to happiness from Herbie Hancock's interview with Wired Blog Network
WN: I've just been listening to some of your latest tracks, and you're still running strong. So many musicians burn out, or can't keep going, and you've been through so many styles, so many incredible records; what do you attribute that longevity to? What's your secret?
HH: Well I have not shut myself off from learning, and the value and beauty of learning and expanding, exploring. That gets my adrenaline going. But I had the great advantage of working with Miles Davis back in the 60s, who, encouraged his young musicians (I was young then) to explore, to take chances, to go outside the box. And I've never forgotten those lessons. So I'm not dependant on staying in the same place. I like the idea of trying something new. I always try to find a way to challenge myself in some way that I feel is valuable to my development. But I have more to give. It's about giving, not getting. That's the bottom line. My feeling is that as long as you keep the flow of ideas going, which means not stopping them when they strike you in your life. Maybe the flow goes through you too, so that it's like a continuous flow, like a river. Then you continue to have ideas. Once you try and start hoarding them and stuff, then you stand in danger of it getting backed up (laughs).
There's only one you. And anyone else who tried to copy you is only a copy. And a copy is never as good as the original. It's not like digital copying. Human copying is not like digital copying, where you can get an exact duplicate of the original. We're not turned into 0s and 1s.