Given the fragility of letters, it’s astounding how many have survived centuries and even millennia. Added to which it seems quite common for people to have purposely destroyed them, whether because they considered them worthless, or wanted to protect themselves or others from the prying eyes of posterity. Jane Austen’s sister Cassandra destroyed many of her letters after her death; John Keats had those of his beloved Fanny Brawne buried with him, meaning that their relationship is conveyed to us in his voice only.
Elsewhere in Gaskell’s writing, she describes sitting down with an elderly lady of the parish and together going through packages of yellowing letters, re-reading them before putting them on the fire, both of them in tears. They included love letters of the older woman’s parents during their courtship.
Thinking about this, in principle I would never destroy a letter, but in practice I have destroyed a handful I wrote and never sent. In some cases the writing was cathartic, and sending the letter would have been unnecessary and even damaging. That was an advantage of the post, there was an unavoidable lapse between writing and sending, allowing some distance and reflection.
‘After tea she resolved to examine a large packet of letters, and pick out those that were to be destroyed.’
Source: Elizabeth Gaskell, North and South (London: John Murray, 1925), p. 414