"It is hard to fill a cup which is already full"

I doubt very much they consider ignorance to be a good thing. But yeah, I never understood that either. Maybe because of his way of life as a human and training within the marines he's already pretty much set in his ways, thus would be hard-pushed to learn another very different way of life, especially one that is.....alien? Or maybe she's saying that humans are too arrogant to be open minded about anything that doesn't fit into their own way of living? Those would be my guesses.