中文：

为什么30年一轮回？这其实是一种历史的周期性规律，也称为“三十年周期”或“康德拉季耶夫周期”。这种周期性的理论认为，经济发展会经历一个从繁荣到萧条，再到复苏的周期性过程，大约需要30年左右的时间。

在这个周期性过程中，不同行业和领域的发展会呈现出不同的特点。例如，在繁荣期，经济增长迅速，市场需求旺盛，企业盈利增加，投资回报率高；而在萧条期，经济增长放缓，市场需求低迷，企业盈利下滑，投资回报率降低。

此外，政治、社会和文化等方面也会受到这种周期性的影响。例如，在政治上，一个国家的领导层可能会在繁荣期获得更多的支持和信任，而在萧条期则可能面临更大的挑战和压力。在社会方面，人们的生活方式和价值观念也会随着这种周期性的变化而发生变化。

这种周期性的现象不仅存在于一个国家或地区的经济发展中，也存在于全球范围内。因此，了解和掌握这种周期性规律对于预测经济发展趋势、制定合理的经济政策以及做出正确的投资决策都具有重要的意义。

英文：

Why does 30 years mark a cycle? This is actually a historical cyclical law, also known as the "30-year cycle" or "Kondratieff cycle". This cyclical theory argues that economic development undergoes a cyclical process from boom to bust, and then to recovery, which takes about 30 years.

In this cyclical process, the development of different industries and fields presents different characteristics. For example, during the boom period, economic growth is rapid, market demand is strong, corporate profits are increasing, and investment returns are high; during the bust period, economic growth slows down, market demand is sluggish, corporate profits decline, and investment returns are reduced.

In addition, politics, society, and culture are also affected by this cycle. For example, in politics, a country's leadership may gain more support and trust during the boom period, but may face greater challenges and pressure during the bust period. In society, people's lifestyles and values also change with the cyclical changes.

This cyclical phenomenon not only exists in the economic development of a country or region, but also exists globally. Therefore, understanding and mastering this cyclical law are of great significance for predicting economic development trends, formulating reasonable economic policies, and making correct investment decisions.