I went on a date with a librarian.

The humor in this joke springs from the delightful clash between expectation and reality, specifically through the lens of a common stereotype. We expect a date to be a relaxed, social interaction, but the punchline brings the very specific professional habit of a librarian into a personal setting. It's situational irony at its best, where the characteristic behavior of a librarian—enforcing quiet—is applied in a completely inappropriate, yet comically understandable, context.

For centuries, libraries have been sanctuaries of silence, places where hushed tones are the norm to allow for concentration and respect for others. Librarians, by extension, have become the iconic arbiters of this quietude, gently (or not-so-gently) reminding patrons to lower their voices. This cultural association is so strong that it's almost a universal understanding.

The joke cleverly plays on this deeply ingrained image, suggesting that the "shush" reflex is so ingrained in our date that it transcends the workplace and pops up even when they're off the clock. It's a lighthearted jab at the idea that some professions become so much a part of us that they can't help but influence our personal lives, leading to a chuckle at the thought of a romantic evening being interrupted by a polite request for silence.