Show a human a closed door, and not matter how many open doors she finds, she'll be haunted by what might be behind it.
A few people liked to paint this drive as a weakness. A failing of the species. Humanity as the virus. The creature that never stops filling up its available living space. ... But Anna rejected that idea. If humanity were capable of being satisfied, then they'd all still be living in trees and eating bugs out of one another's fur. Anna had walked on a moon of Jupiter, She'd looked up through a dome-covered sky at the great red spot, close enough to see the swirls and eddies of a storm larger than her home world. She'd tasted water thawed from ice as old as the solar system itself. And it was that human dissatisfaction, that human audacity, that had put her there.
Looking at the tiny world spinning around her, she knew one day it would give them the stars as well.
James S. A. Corey, Abaddon's Gate