There are a lot of competing factors that drive language change. Probably the largest force is sound efficiency - speakers merge, elide, and otherwise mess with sounds in order to make speaking take less energy, or make it possible to talk faster (think runnin' instead of running). This increases efficiency for a while, until homonyms take over and meanings become too frequently ambiguous. (Think of the cot/caught merger, although that one probably confuses no one.) Then, speakers start to use lots of periphrasis and such, and eventually big compound descriptions get lexicalized as new words and grammatical structures. This makes the language more complex as it originally was. Thus language evolution often happens in cycles like this on the order of centuries.
The major factor that influences the equilibrium of this cycle is number of speakers and number of adult learners. Languages with lots of speakers change more slowly, and tend to be getting simpler especially if they are assimilating new speakers (think English as it is becoming the global lingua franca.) Small insular languages with few second language learners tend to get more complex and idiosyncratic.
The major factor that influences the equilibrium of this cycle is number of speakers and number of adult learners. Languages with lots of speakers change more slowly, and tend to be getting simpler especially if they are assimilating new speakers (think English as it is becoming the global lingua franca.) Small insular languages with few second language learners tend to get more complex and idiosyncratic.
Source: studied linguistics for a few years.