Certain lines survive long after the programme that produced them has been forgotten. The transition happens when a phrase stops being a reference and starts being a tool.
The phrase has to fill a gap in ordinary speech
Lines that spread furthest tend to express something the language handles awkwardly, such as a specific kind of resignation or a particular type of dismissal.
Where an existing phrase already does the job well, the new one has no room to establish itself and remains a quotation.
Usefulness is the filter. A line can be enormously popular within a fandom and never escape it, because outside that context there is no occasion to say it.
Delivery matters more than the words
Most durable catchphrases carry a distinctive rhythm or intonation, which is what makes them recognisable when repeated by someone else.
That pattern travels intact even as the wording drifts, and people often reproduce the cadence more accurately than the original text.
Lines that depend on the plain meaning of their words rarely last, because there is nothing to reproduce beyond the sentence itself.
Repetition inside the show builds the association
A phrase used once is a good line, while a phrase used across many episodes becomes attached to a character in a way audiences internalise.
Writers often notice a line working and deliberately repeat it, which accelerates the process but also risks exhausting it before it escapes.
The strongest examples are usually those that were repeated because they were useful to the character rather than because they were noticed as catchphrases.
Detachment happens when the source is forgotten
The final stage is a phrase being used by people who have never seen its origin and would not recognise it if shown.
At that point the line has entered ordinary vocabulary, and its meaning can drift away from the original joke entirely.
Some widely used expressions began as television lines decades ago and are now used by speakers who assume they are simply idiom.
Overuse ends most of them
Phrases that spread rapidly also saturate rapidly, and heavy use makes a line sound tired long before it has established itself.
Advertising accelerates this, since a phrase attached to a campaign is repeated far past the point where anyone finds it amusing.
The lines that survive are usually those that spread slowly enough to become habitual before they became annoying, which is not a process anyone can engineer.