Jake is shown to be cynical about his relationship with Brett, despite being infatuated with her for the majority of the novel, but when she discusses the life they could have led, he acts unbothered: “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?” Loc3655
He mocks this hypothetical perspective Brett mentions, by talking about it like it's some kind of dream state that will never be achieved.