Answer the following questions briefly:

What prompts the author to write the last letter?

The author is prompted to write the last letter so that the set of letters he has been writing to his daughter are provided with some sort of flourish in the end, and it does not seem too abrupt and ending for them. Besides, he feels that the joy of writing has taken the better of him during these harsh days, and although there is no need for him to pen down yet another letter, he simply cannot let go of the opportunity.