Whether or not a country has an official standard for its national languages set and maintained by an academy or in the form of dictionaries and prescriptive grammars, the actual language of its speakers keeps changing, not only from generation to generation,
but also during the lifetime of a single person. Language being a set of habits maintained chiefly through the interaction between members of a speech community, it will change if the frenquency of intercommunication is diminished. Therefore, instead of asking why does a language change, a more natural question is to ask why should a language remain as stable as it is? Habits change, things are forgotten, people drift apart, and it is remarkable that language does not change faster than it does.
(Language and Symbolic Systems, Yuen Ren Chao)