Interesting.  I'd be a little wary about drawing lessons from the likes of Nietszche and Darwin though.  Darwin studied birds and turtles, and came up with a very important scientific advancement.  However, the human mind being unfathomably complex, I think the lessons to be drawn about personal happiness and fulfillment from the general outline of long-term species evolution are nil.  In my opinion, whatever your philosophic or religious outlook, there is great wisdom to be had from religion and ancient philosophers (especially Aristotle).  Human happiness is certainly one of the most difficult and elusive concepts to think about.  I will be honest, I think Nietsche is one of the most disastrously wrong thinkers, who has had an utterly atrocious effect on the state of the human mind. It astounds me that liberal, generally kind-hearted people can excuse so many of the despicable things he wrote.  The link between him and Hitler is often disputed, but in my mind it is undeniable.  Nietszche was no anti-Semite, but in terms of general outlook, Hitler did exactly what Nietszche called for.  He set out to make himself a superman, stronger and wiser than everyone else (Zarathrustra anyone?).  He felt no compunction or lingering Christian love of pity for mankind, only contempt for the vast majority of mankind.  He crushed and persecuted the weak (something that Nietsche explicitly exhorted by praising cruelty and repeatedly disputing the inherent worth of all men). Hitler's version of Nietschism was obviously far more vulgar than the master's, but a close look at Nietsche reveals a thinker who tried to tear down all old morality, even and especially the aspects of it that called for respect for humankind and protection for the weak.  That was a bit of a rant, but I suppose my main message is, Nietsche has little to say about human happiness.  I agree with your point about challenging yourself, but in no way is this new to Nietsche.  This idea goes back at least to the ancient Greeks, and Aristotle in particular, who located happiness in the struggle for a life of virtue.  If anything, Nietsche perverted the idea into a notion of "power."  If we consider history, it is instantly obvious that the powerful have not always been just (definitely an understatement).  Power is certainly important, but the pursuit of it alone leads not to happiness but to a debasement and perversion of the human soul.