Polite laughter can keep a joke alive after nobody finds it funny.

Politeness has a preserving effect that nobody intends. The laugh given out of warmth is indistinguishable, from the outside, from the laugh given out of amusement, so the person telling it hears encouragement and tells it again. Whole routines survive for years on that misreading, in families, in offices, in friendships. Nobody is being dishonest exactly, and yet a small false signal is quietly being maintained by everyone present.

Consider what you might be keeping alive with the same reflex. It is usually kinder, eventually, to let a silence happen than to fund another decade of the same story.