Our primitive ancestors spread from east Africa to all over the planet, and since then, we've never stopped moving. We're running out of fresh territory on Earth, so the only way to meet this ancient urge is to find somewhere new to go -- whether it's making brief jaunts to the moon as a tourist, or signing up for an interstellar voyage that will take multiple generations.
In a 2007 speech, former NASA administrator Michael Griffin differentiated between "acceptable reasons" and "real reasons" for space exploration. Acceptable reasons would be issues like economic benefit and national security. But real reasons include concepts like curiosity, competiveness and monument-building.
"Who among us does not know the wonder and mystery and awe and magic of seeing something, even on television, never seen before, an experience brought back to us by a robotic space mission?" Griffin added that "when we do things for real reasons as opposed to acceptable reasons, we produce our highest achievements."