As the introduction to the Loeb edition says, In February 45 [a year before he wrote De Senectute] the death of his adored and only daughter drove him into a frenzy of writing in an effort to forget his grief.
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His only daughter died, which led him to write as a distraction for his grief.
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His account of coping with the death of his daughter was not recorded, but is said to have been noted during an interview.