Why can't I understand what Zhejiang people say? Why are there language differences in different regions?

Chinese dialects, commonly known as local dialects, are only used in certain areas. It is not another language independent of the national language, but only the language used in some areas. Most dialects in modern Chinese have gone through a long process of evolution and gradually formed. There are many factors that form Chinese dialects, such as social, historical and geographical factors, such as population migration, mountain and river barriers and so on. There are also factors that belong to the language itself, such as the imbalance of language development and the mutual contact and influence between different languages. Among several dialects of modern Chinese in China, the northern dialect can be regarded as the development of ancient Chinese in the vast northern region after thousands of years, and the rest dialects are gradually formed by the continuous southward migration of northern residents in history. The early Jiangnan area, mainly inhabited by the Guyue nationality, used Guyue language, which was far from ancient Chinese and could not speak. Later, the Han people in the north went south several times on a large scale, bringing ancient northern Chinese in different periods, which spread to various regions in the south of the Yangtze River and gradually formed six dialects that were obviously different from each other. There are three reasons for dialect differences: first, before the contact between northern Chinese and southern ancient Vietnamese, there were their own regional dialects; Second, China people in the north go south at different times, and naturally China people go south at different times. Third, the southern dialect developed in a certain unique environment. In the development of Han society, there have been different degrees of differentiation and unification, which gradually led to the emergence of Chinese dialects. Our country has a large population and complicated situation, and the dialects that can't be spoken are divided into regions. According to the modern popular distribution, modern Chinese dialects can be divided into seven dialect areas. Namely Northern Dialect (Mandarin), Wu Dialect, Xiang Dialect, Gan Dialect, Hakka Dialect, Min Dialect and Yue Dialect. At the same time, in complex dialect areas, some dialects can be further divided into several dialects (sub-dialects), and even can be divided into "dialects". These dialects are dialects that know every place (a certain thing, a certain county, a certain town, a certain village), so they are called local dialects. For example, Guangzhou dialect and Changsha dialect are distributed in Shanghai, east of Zhenjiang in Jiangsu Province, south of the Yangtze River (excluding Zhenjiang), a small part of Nantong and most parts of Zhejiang. Suzhou dialect is a typical representative of Wu dialect. The number of users accounts for about 8.4% of the total population and 7.2% of the total population of Han nationality. Wu dialect is divided into Taihu dialect (northern Wu dialect, including southern Jiangsu, Shanghai and Zhejiang Huzhou, Jiaxing, Hangzhou, Shaoxing and Ningbo, represented by Shanghai dialect or Suzhou dialect), Taizhou dialect (Zhejiang Taizhou area), Wuzhou dialect (Zhejiang Jinhua area), Chuqu dialect (Zhejiang Quzhou and Lishui area), Oujiang dialect (Zhejiang Wenzhou area) and Xuanzhou dialect. Among them, southwestern Anhui and western Zhejiang are influenced by Gan dialect, while southern Zhejiang retains many features of ancient Baiyue dialect, so that Taihu Wu dialect cannot be regarded as a typical Wu dialect.