Why is C.S. Lewis so concerned with being locked into a wardrobe?

Question

In The Lion, The Witch, and the Wardrobe author C.S. Lewis makes five references to the characters making sure to not latch the wardrobe from the inside:

[Lucy] immediately stepped into the wardrobe […], leaving the door open, of course, because she knew that it is very foolish to shut oneself into any wardrobe. [Chapter 1]

[Lucy] had, of course, left the door open, for she knew that it is a very silly thing to shut oneself into a wardrobe. [Chapter 1]

[Lucy] did not shut [the door] properly because she knew that it is very silly to shut oneself into a wardrobe, even if it is not a magic one. [Chapter 3]

[Edmund] jumped in and shut the door, forgetting what a very foolish thing this is to do. [Chapter 3]

Peter held the door closed but did not shut it; for, of course, he remembered, as every sensible person does, that you should never never shut yourself up in a wardrobe. [Chapter 5]

This is mentioned enough times that it seems to be more than a passing bit of advice, although I imagine at least one purpose was to discourage any impressionable young readers from getting trapped and suffocating inside a wardrobe in their own imaginative attempts to reach Narnia. Is there any other known reason for harping on this rule?

Answer 1

According to an article on the CS Lewis Institute website:

The actual wardrobe that prompted the stories was one made by Lewis’s grandfather and was in the family home in Belfast. Later, it was moved to Lewis’s home at Oxford and now resides at the Wade Center, at Wheaton College. One of C.S. Lewis’s cousins, Claire, remembered occasions when various cousins along with “Jack” (C.S. Lewis) and his brother Warren, would climb into the wardrobe while young Jack would tell them stories he had invented. It is interesting to note that Lewis mentions a few times that “it is foolish to shut oneself into any wardrobe” perhaps because he always kept a crack of light when he told his stories and also because he was warned. When Lewis sent a draft of LWW to friend Owen Barfield, Barfield’s wife Maud was concerned lest children read the story and accidentally lock themselves in a wardrobe. So Lewis added five warnings to LWW. The wardrobe is such a vivid image that one Oxford boy, after reading the book, chopped a hole in the back of the family wardrobe trying to get to Narnia.

Note that the Barfield's daughter Lucy was Lewis's goddaughter, and Lucy Pevensie is her namesake.

This information seems to originate from the book Into the Wardrobe: C.S. Lewis and the Narnia Chronicles by David Downing, which says (p. 35):

When Lewis sent Owen Barfield a draft of The Lion, the Witch and the Wardrobe, Barfield’s wife Maud was concerned lest children read the story and accidentally lock themselves inside a wardrobe. Lewis took this cautionary note to heart and ended up adding five warnings to The Lion, the Witch and the Wardrobe about not closing the door and locking oneself in. After the story was published, a little boy in Oxford took a hatchet and chopped a hole in the back of a family wardrobe, hoping to find his own way into Narnia.

The first part of this (Maud Barfield's concern) comes in turn from Walter Hooper's footnote in Collected Letters (volume II, p. 942):

66 Barfield’s wife was concerned about the inhumane trapping of animals, and when she read the manuscript of The Lion, the Witch and the Wardrobe, she mentioned this to Lewis, as well as her fear of children locking themselves in wardrobes. Because of this Lewis included a warning in ch. 1 of the story: ‘[Lucy] had, of course, left the door open, for she knew that it is a very silly thing to shut oneself into a wardrobe.’ In fact, there are five such warnings in the first five chapters of the book.