Why is the song of Preface to Lanting so sad?

The artistic conception conveyed is profound and euphemistic. The artistic conception conveyed by the song Preface to Lanting Collection is profound and tactful, which makes people feel so sad. The background of this song is that Wang Xizhi, a writer in the Eastern Jin Dynasty, selected five famous calligraphy works at that time, Preface to Lanting Collection 15, and held a grand banquet of Lanting Elegant Collection. At the same time, this song also expresses the author's feelings about the passage of time and his short thinking about life, which will touch the heartstrings of the audience.