From beginning to end, the Book of Changes is inseparable from the words Yin and Yang, which are like two forces that transform each other, opposing and unifying. When the yin reaches the pole and the yang reaches the pole, they will all change in the opposite direction, which is the truth that the pole of things must be reversed. It's like a cloudy cloud in the sky that indicates that rain is coming, but a strong wind blows, the dark clouds disperse, and the sun shines down, turning from yin to yang.
The opposition and unity of yin and yang is the essence of the philosophical concept of the Book of Changes, covering important areas such as heaven, heaven and man, humanity, and dialectics. What I Ching talks about is the way to balance yin and yang, and how to achieve this harmonious state is the "righteousness" that I Ching wants to tell us. 
Although the word harmony is simple and easy to understand, it is actually difficult to achieve. For example, the development of society depends on the continuous reproduction of human beings, the combination of yin and yang, and the endless growth. The harmony of society requires the harmony of all small units to be built together. Whether it's a big country or a small family, harmony is needed. National harmony requires democracy and the rule of law, fairness and justice, honesty and fraternity, security and order, so that people's morality and values can be improved, income can be stable, life and work in peace and contentment, and the happiness index can be improved. The harmony of the unit requires a harmonious relationship between leaders and employees, unity, sincere cooperation, and the continuous development and growth of the company. Family harmony requires the love of husband and wife, the lively and lovely children, the health and longevity of the elderly, and the harmony of family members.
However, harmony is not achieved overnight, and requires continuous efforts and pursuits. This is exactly what the I Ching teaches us: to seek harmony in the mutual transformation of yin and yang, and to find unity and balance in contradiction and opposition.
Harmony does not mean that there are no contradictions, but that when there are contradictions and different opinions, we should seek common ground and seek differences and try to resolve them. This is exactly the principle of yin and yang changes mentioned in the I Ching. It is inevitable that there will be quarrels between husband and wife, but as long as there are no issues of principle such as cheating, it is common for the bedside to quarrel at the end of the bed. Crying is yin, laughing is yang, isn't it the truth that yin turns yang?Occasionally creating some small contradictions, so that a small ripple in a peaceful life, can promote the relationship between husband and wife or couples. In the past, I had a neighbor who respected each other and never quarreled, but one day I suddenly heard that the two were divorced. Behind the peaceful life is a volcano, and the emotions of both of them have no place to vent, and they will inevitably erupt eventually. Therefore, when they find that the other party is in a bad mood, couples can go out more often, such as traveling. 
Traveling can be described as a panacea to solve the conflict between husband and wife. You see those couples who quarrel and go out often return with joy at the end of the journey. From haze to sunshine, this is the principle of yin and yang balance. However, as a husband, it is important to remember that you should not do anything to your beloved wife at any time. You are a big husband, and you should have the bearing and self-cultivation of a gentleman. Of course, there are as many discordant families as autumn leaves, and divorce is not uncommon. If couples can learn Xi I Ching, understand the true meaning of the way of yin and yang, and understand what true harmony is, then the divorce rate in this society will be much reduced. A smart man is not macho, but a combination of hardness and softness, and is good at making his wife happy while making her feel dependent. A wise woman gives her husband face outside, and when she returns home, she shows a gentle and coquettish side, firmly tying her husband's heart. And some women dress up flamboyantly when talking about their partners, but after getting married, they no longer pay attention to dressing up, and they lose their gentleness when they speak. In fact, men also need to create some romance once in a while and repay each other's love with practical actions. What we yearn for is a harmonious society and a big family. In this big family, man and nature live in harmony. However, yin and yang always grow and disappear from each other, and opposites are unified. Just like water and fire, north and south, east and west, water and fire are opposites, and the downward flow of water is in stark contrast to the upward flow of fire. But from another perspective, we will find that yin and yang are unified. If we switch the position of water and fire in the midst of fire and water, then we have formed a combination of fire and water. This is the embodiment of the unity in opposites, the yang in yin and the yin in yang. For example, some men are strong on the outside, but they also have a careful and gentle side;Some women have bold and decisive personalities, but they also have feminine softness and resilience.