What are the reasons for varieties in language?

What are the reasons for varieties in language?
Various civilizations developed around the world over time, and each had its own vocabulary. As the groups interacted with each other, spellings and concepts were exchanged. But without constant exposure to other regions, the speakers (and writers) codified their own languages to express more complex concepts in a generally accepted form for their societies. So the Roman form of Latin, for example, was subject to regional adoption and became part of French, Spanish, Catalan, and Portugese (and eventually English) as well as Italian. These major languages are still further divided into ethlic and locational dialects, where specific local cultures were not shared by a majority of the populace. In China, for example, there were (and are) a dozen distinct historic dialects within the country. Individuals living between two areas of different dialects may adopt language elements from two or more adjacent regions.