Why do people call marriage "a good marriage in Qin Jin"?

The Friendly Relations between Qin State and Jin State ―― Marriage of the Two Families

During the Spring and Autumn Period, the State of Qin (now Shaanxi) and the State of Jin (now Shanxi and southern Hebei) were two neighboring countries. The ruling groups of the two countries are intrigued for hegemony, and the contradiction is very sharp. Sometimes they send troops to fight each other, fighting and killing. On the other hand, for their own interests, they sometimes unite with each other, use each other, and even marry each other to form close in-laws relations.

Qin Mugong, one of the five tyrants in the Spring and Autumn Period, is married to Jin Xiangong's daughter; Jin Xiangong's son Jin Wengong was also one of the five tyrants in the Spring and Autumn Period, and his wife Wenbo was Qin Mugong's daughter.

Therefore, despite many contradictions, Qin and Jin were married again and again, which is quite prominent in the relationship between countries.

Because Qin Jin was married for life, later generations called the marriage between the two families "mutual Qin Jin" or "the goodness of Qin and Jin".