Unfortunately, I can report that I have done all of my best work in my career when I was extraordinarily anxious, feeling like I was holding to the side of the cliff with just my fingernails. Whenever things have started to get too comfortable, I found that hubris has disrupted my successes.
It's a sad thought, but I see absolutely no reason to believe that the natural state of humanity is to be happy (or, more specifically, content). The Environment of Evolutionary Adaptiveness [0] seems like a pretty harsh place, and that's also where I've found most brains function best.
[0] http://en.wikipedia.org/wiki/Evolutionary_psychology#Environ...