Hemingway takes his the Old Testament Book of Ecclesiastes 1:15: The sun also ariseth, and the sun goeth down, and hasteth to his place where he arose.
Carlos Baker, Hemingway's biographer, has said that the author intended the novel as tragedy; lives are short, days are numbered, we all return to dust. There is great sorrow in Hemingway's claim of being a part of a "lost generation." Not knowing if one has a place or purpose in the world is depressing, to say the least. The post-World War I generation of which Hemingway was a part found little to hope for in a world proven capable of such brutality, nor acceptance for artists who had no interest in the status quo.
All of the characters are lost, from Robet Cohn who has missed his opportunity to hit the big time, to Jake and Brett who are in continual miscommunication and pain. Even Romero, the bull-fighting champ, will soon find his time over.
In one of the most poignant passages of the novel, Brett and Jake have the following exchange in the final lines:
"Oh, Jake," Brett said, "we could have had such a damned good time together."
Ahead was a mounted policeman in khaki directing traffic. He raised his baton. The car slowed suddenly pressing Brett against me.
"Yes," I said, "Isn't it pretty to think so?"
Jake knows that their days have past and are never to be recovered. It is "pretty to think" it might have been different, but not possible.