One reason I don't drink is that I want to know when I am having a good time.

The quip frames abstinence not as dour self-denial but as a preference for clear-minded pleasure. By implying that alcohol can blur the line between genuine enjoyment and chemically induced cheer, the speaker claims a kind of sovereignty over experience: a “good time” should be consciously recognized, not merely felt in retrospect or assumed because one has been drinking. The humor depends on reversing a common justification for drinking (to have fun) into an argument against it (to be sure the fun is real). Attributed to Nancy Astor—famed for sharp, epigrammatic repartee—the line also functions as social commentary on elite conviviality and the performative nature of party-going.