Book 1 Chapter 15's title foreshadows the Chinese subtitle of Book 2.
Origin of the Chapter's Title
The title 思往事，易成伤 (sī wǎng shì ， yì chéng shāng) are lines 5 and 7 from the poem 訴衷情 (sù zhōng qíng) by Ouyang Xiu (歐陽修).
訴衷情 (sù zhōng qíng) can be translated as Telling of the Sorrowful Love or A Recital of a True Heart and is the Chinese subtitle of Book 2.
The poem is about a man missing a woman after they have parted and the sorrow that accompanies as he touches her make-up and retraces the eyebrows he used to draw for her.
思往事，易成伤 can be translated as these:
- Recollecting bygones predisposes me to suffer pain.
- Thinking of the past is easily sorrowful.
The context of the line in its passage:
A translation of the passage:
Feeling surrounded in sorrows of parting,
The brows I drew were long like mountain ranges.
Recollecting bygones and bemoaning loss of time predispose me to suffer pain.