That quote is from a play, and was written by a character who is a revolutionary and an anarchist.

In a meta-sense, I don't think G.B. Shaw intended this quote to be taken as an absolute truth by the audience.

And, even taking the quote itself at face value (pretending the fictional character who wrote it was a real person), I think it's referring mostly to political progress. Indeed, most times that political institutions have been upended, it was by people who were not happy with the status quo. But I think that's a different sort of unhappiness from the unhappiness of not being satisfied with your life.