I think that on the one hand, it is influenced by China's cultural ethics. In China's cultural tradition, it advocates gentleness and subtlety, and disapproves of passionate things, especially in terms of men's and women's feelings. Therefore, most of the poems contain feelings that are not revealed. In addition, with the feudal etiquette system becoming more and more strict and widely accepted, the combination of men and women is more "the orders of parents and the words of matchmakers", and there is no chance to meet each other before marriage. How can there be love? Even if they really have feelings for each other, they dare not announce them, otherwise they will not be accepted by the etiquette of the society at that time. As for the harmony between married couples, it is a beautiful talk, which can of course be promoted. The feelings of unmarried men and women can be openly and boldly expressed in The Book of Songs, but there are fewer and fewer such words in later generations. But on the other hand, this kind of etiquette is contrary to human nature. Young men and women are also longing for love, or have a hazy affection for the opposite sex, so there are many poems about the disappointment and pain of missing their lovers or the love of childhood friends being destroyed.
On the other hand, tragic things are really moving, which are easy to cause * * * and may be circulated for a long time.