Languages change for a variety of reasons. Historically, one of the main factors involved in shaping a language's evolution has been the interaction between new groups of people as a result of movement to new areas. When one culture encounters another, the languages of both groups will be impacted as the people learn to communicate with each other. New words and phrases will be created, adopted or adapted to allow for understanding between people. New pronunciations may develop as people listen to each other and are influenced in their speech patterns by what they are hearing.
Language also changes in response to new circumstances or developments. Think of all the new terminology and phrases that have come into common usage during your lifetime as a direct result of the expansion of technology! Computers and the internet are also expanding the sharing of culturally-based influences mentioned above.