Why did Gustav Mahler avoid calling Das Lied von der Erde his ninth symphony?

Gustav Mahler avoided labeling Das Lied von der Erde as his ninth symphony due to a deep-seated superstition regarding the "curse of the ninth." Drawing an analogy from the lives of Ludwig van Beethoven and Anton Bruckner, Mahler believed that a composer’s ninth symphony would inevitably be their final completed work.1

By withholding the number from Das Lied von der Erde, Mahler hoped to circumvent this perceived fate. When he later composed his actual Symphony No. 9, he remarked half-jokingly that the danger had passed because the work was "really the tenth." Ironically, his superstition proved persistent; the Symphony No. 9 became the last symphony he completed, and his Symphony No. 10 remained unfinished at the time of his death.1