Why is that funny?

I have a friend, who, in rehearsal with a particularly uninventive and by-the book-director... no, traffic cop....when the latter screamed at another hapless and trangressing performer who had dared to "move on the punchline," WHAT IS THE FIRST RULE OF COMEDY?!?!?!?!?, brought the house down by muttering, "I dunno.... a red nose is a funny nose?"

For all the contempt we may have had for him, it must be admitted that the bad director had something right, there are "rules" of comedy.
But what are they?

Himslef, when a director delivers the "There are two speeds for comedy, fast and faster," cliche, is wont to request, "Somebody please inform Jack Benny."

A review of a movie I had, and still have no interest of seeing, (so why bother reading the review? hmmm....) has this gem of a put down:

[The] sketch ... has repeated itself enough times to have acquired a reputation for being humorous.

And in the scant attention I have paid to Saturday Night Life for the past decade or so, this seems to be their stock in trade, repetition, so that instant recognition of a character, phrase or situation brings on cheers and whoops by the studio audience, masking the fact that what then occurs is deeply unfunny.

Monty Python was, and David Letterman is beyond skilled in repeating the unhumorous until it become hysterically funny, (the latter, in some kind of meta-humor that mocks perseverance, obliviousness, a host of things...)

But why?
How do they do this? Since it must be admitted that most people can't, and that most things do NOT become funnier with repetition.

(Monty Python also used to do a send-up of academe that with a Ben Stein-level boring explanation of the mechanics of humor by dead-pan experts using custard pies, so perhaps this is something that defies analysis and on which I should not ponder...)