Why is Li Yu's "Hui Huan" about the sadness of parting?

Ci is a kind of poetic style, also known as "Quzi Ci", that is, lyrics. This was originally a poem written with music singing. Because it is sung to music, every word has or at least once had a score. Every musical score must belong to a certain palace tune (similar to today's C tune and G tune), with a certain melody and rhythm, and the sum of these things is the tone of words. Every piece of music has a name (such as Xijiang Moon and Qingping Music), and this name is called epigraph. The epigraph has little to do with the content of music or words. When writing lyrics, the author only considers which form of tone is more in line with his own needs, and can completely ignore the original intention of the name of the epigraph and the content of the original lyrics. So generally speaking, the epigraph only shows which ready-made music score the author uses to fill in the words. "Yinghuan" is a epigraph name. So "Meeting" can be used to write farewell, and "Shuang Yan" can also be used to write farewell.