According to Deutscher, the motives for language change are:
“Economy refers to the tendency to save effort, and is behind the short-cuts speakers often take in pronunciation.”
“…when these short-cuts accumulate, they can create new sounds, just like a new footpath cutting through a field.”
E.g. “wasn’t”, “innit”, “bruv” – these are instances where our language is getting more efficient – not more sloppy and careless – as Jean Aitchison’s damp spoon syndrome prescriptivists would have us believe.
…the constant battle against cliché and stale language / language bleached of meaning – “good” wasn’t good enough anymore, it began to lack effectiveness, so people resorted to metaphors, such as “brilliant”, “awesome”, “incredible”, “unbelievable”, “amazing”, “fabulous”… which made our language more effective – but only for a while, eventually these fresh metaphors got stale, became clichés and eventually died. As dead metaphors we don’t see them as figurative any more. And we are in need of new fresh metaphors to keep our language fresh and effective.
Compare this to the infectious disease assumption?
“Expressiveness refers to speakers’ attempts to achieve greater effect for their utterances and extend their range of meaning. One area in which we are particularly expressive is in saying “no”. A plain “no” is often deemed too weak to convey the depth of our unenthusiasm, so to make sure that the right effect is achieved, we beef up “no” to “not at all”, “not a bit”, “no way”, “by no means”, “not in a million years”, and so on.”
“…the results of this hyperbole can often be self-defeating, since the repetition of emphatic phrases can cause an inflationary process that devalues their currency.”
“…the mind’s craving for order, the instinctive need of speakers to find regularity in language.”
Deutcher points to the reduction in irregular plural nouns in the English language: whatever happened to “shoen” and “housen”? As in “brethren” and “oxen”? Why do all new plurals now take the regular “-s” ending? And why have we gotten rid of the irregular “-en” ending? How come “snuck” was the last ever irregular past-tense form to sneak in to our language? We don’t say “he gugled” we say “googled”. This is the direct opposite of the Crumbling Castle Theory with which Aitchison mocks some prescriptivists.