Why do languages change?

[Forbes]

there is no doubt that language changes. We read a lot about the ways in which languages change, but not why they change. In the modern world one can see that new things need new words, but why in an ancient stable society would not only the lexicon but also the grammar change over a period?

One book suggested that there would only seem to be two possibilities both of which were unlikely. One is that everyone at the same time decides to change something; the other is that one person makes a changes and everyone else decides to run with it.

[Daniel]

To answer this in the most abstract way possible, consider that languages don't really exist: they're just out impressionistic snapshots of language usage at a particular moment in time. In that sense, how could languages not change?

As for the process of language change in terms of usage, that can't be fully explained linguistically, but must be considered sociolinguistically. A particular accent may be perceived as desirable not because of the accent itself, but because of who speaks that way, and the same with vocabulary, and also grammatical features. There are other factors too, including acquisition and contact, but that's the most general answer. That's why we can't predict the future of a language, just trace its history. There are also some ways to look at this strictly linguistically (but as I said, not for a complete answer), including especially frequency of usage (i.e. irregular verbs or plural nouns, etc., are almost always among the most frequently used in a language), and there may be some external factors as well such as phonetic motivations for certain sound changes.

Beyond that, it's not how but just what changes.