He had no conscious memory of what he had seen; but that night, as he sat brooding at the entrance of his lair, his ears attuned to the noises of the world around him, Moon-Watcher felt the first faint twinges of a new and potent emotion. It was a vague and diffuse sense of envy – of dissatisfaction with his life. He had no idea of its cause, stili less of its cure; but discontent had come into his soul, and he had taken one small step toward humanity.
2001: A Space Odissey – Arthur C. Clarke
Why do we do what we do? Dissatisfaction.