“Because I am still a little girl who believes in Santa and the tooth fairy and you.”

Yes, Virginia, there is a Santa Claus (1897)
Context: Not believe in Santa Claus! You might as well not believe in fairies! You might get your papa to hire men to watch in all the chimneys on Christmas Eve to catch Santa Claus, but even if they did not see Santa Claus coming down, what would that prove? Nobody sees Santa Claus, but that is no sign that there is no Santa Claus. The most real things in the world are those that neither children nor men can see. Did you ever see fairies dancing on the lawn? Of course not, but that's no proof that they are not there. Nobody can conceive or imagine all the wonders there are unseen and unseeable in the world.
You may tear apart the baby's rattle and see what makes the noise inside, but there is a veil covering the unseen world which not the strongest man, nor even the united strength of all the strongest men that ever lived, could tear apart. Only faith, fancy, poetry, love, romance, can push aside that curtain and view and picture the supernal beauty and glory beyond. Is it all real? Ah,, in all this world there is nothing else real and abiding.
No Santa Claus! Thank, nay, ten times ten thousand years from now, he will continue to make glad the heart of childhood.

Source: Rant

Source: Peter and Wendy (1911), Ch. 3

"Your Dictionary".
Apple Venus Volume 1 (1999)

Source: Runaways, Vol. 1: Pride and Joy

Variant: The main reason Santa is so jolly is because he knows where all the bad girls live.

Source: And the Mountains Echoed

Cosmopolitan, January 1999, on embarrassing dates.