I think there are a variety of reasons, most of which have been mentioned. Also luck -- simply being in the right place at the right time.
Oh, but that simply is outward excellence. What about inward excellence? Take a look at Mihaly Csikszentmihalyi's book "Flow" and you will see that humans can have "peak experiences" no matter what path in life they have chosen/been thrust into. It's a fascinating work.
Bradley