Comedy is the most unforgiving genre. For a novelist, poet, or scholar, an admixture of mediocrity need not be damning. An occasional prosaic thought can pass unnoticed amid the greater thoughts around it, just as no one notices the mulch in a bed dense with flowers. Even Milton writes dull verses sometimes, which we enjoy reading because they let us rest from interpretation.
But for a comedian alone among writers, anything that does not hit, misses. A joke that falls flat cannot fall back to being filler; no lower purpose redeems it. A poor punch line in a stand-up act, instead of fading in the surrounding laughter, creates the awkward silence that everyone in the audience remembers. Every comedic utterance that is not a triumph is a failure.
A comedian needs courage besides humor.