A really simple expression of the human condition is that mankind easily visualizes (and wants) more than he has and can also usually get by (perhaps more happily) with less than he has. That balance is his challenge, or his condition. Wikipedia has more to say: http://en.wikipedia.org/wiki/Human_condition.
Errol Morris quotes this from Robert McNamara, in a 1966 speech in Montreal (delivered while he was still secretary of defense):
Who is man? Is he a rational animal? If he is, then the goals can ultimately be achieved. If he is not, then there is little point in making the effort. All the evidence of history suggests that man is indeed a rational animal but with a near infinite capacity for folly. His history seems largely a halting, but persistent, effort to raise his reason above his animality. He draws blueprints for utopia, but never quite gets it built. In the end he plugs away obstinately with the only building material really ever at hand: his own part-comic, part-tragic, part-cussed, but part-glorious nature.
Its the endless juggling of personal morality, loyalty, political possibility and the caprice of history. If he failed, it is because he tried to bring his idea of rationality to problems that were bigger and more deeply irrational than he or anyone else could rationally understand. For me (Errol Morris), the most telling moment in my film about Mr. McNamara,The Fog of War,is when he says, Perhaps rationality isnt enough. His career was built on rational solutions, but in the end he realized it all might be for naught.
Here's another quote that bears on mankind as the 21st century opens up (thanks Suzy).
"The intuitive mind is a sacred gift, and the rational mind mind is a faithful servant. We have created a society that honors the servant and has forgotten the gift." ~ Albert Einstein