If anything, Homo Sapiens was designed (read evolved) to be flexible because species that hard-coded their responses to the environment went extinct when the environment changed. So the only thing that could be coded into human behavior was to try to be "better". Better than what was left intentionally blank. At least, that's how I see it.

There is a popular TED talk on motivation, he postulates that growth is one contributing component of motivation. This may be his anecdote, but this guy has helped many people get back their motivation. http://www.ted.com/talks/dan_pink_on_motivation.html