Get your calculators

Mathematically:

The humour rating and the entertainment value of a joke (or a try-to-be-one) decreases due to a number of factors:

the closeness/familiarity between the joker and the jokee increases
the amount of time spent together increases
the amount of jokes said before is high

*There is a law of diminishing humour pertaining to the situation. As time goes by and the amount of jokes increases, the amount of humour and entertainment perceived increases significantly, and perhaps enjoy economies of scale due to the "splash-over" effect of previous jokes, but up to a certain peak, it will begin to drop in value and can often go into negative.

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I should try to submit this to get a Nobel.