In the last words of this novel, Hemingway delivers a memorable and hard-hitting diagnosis of his generation: "Isn’t it pretty to think so?" The speaker, Jake, is referring specifically to the idea that he and Brett, his romantic interest, could have had "a damned good time together."
More generally, however, Hemingway expresses the sense of hopelessness and resignation that he sees in the world around him. Basically, it’s saying that everyone has their hopes and dreams—but they can’t be fulfilled. The best anyone can do is wistfully (or cynically) indulge in the fantasy that these dreams could have been possible in some alternate universe.