This thread is on why languages change, not on how they change.
Yes,
why indeed?
there's definitely
no simple answer to that question, and there won't be - ever (that's my opinion, of course - no scientific axiom ;-)
however, some reasons have been established by linguists, and I might add, none of them (I'd say) is undisputed (meaning: there's never one single reason, there always are more - it's not as easy as that):
beforehand: I would like to differentiate between simple 'forces' of change (meaning: factors involved in language change) and 'reasons' for change (meaning: the factors = forces
responsible for change taking place at all):
- simplification: yes, it plays its role, simplification in the grammatical sense, but it is not so easy that one could say simplification goes towards isolating languages (such as English and Persian), simplification can also lead towards inflection, meaning linguists all over the world haggle over which would be the most 'simplistic' structure, and if you ask for my opinion, then simplification ist just a force but hardly the reason (the latter it may be in certain conditions, but this is not the rule, e. g. English with mixed population of Anglo-Saxons and Normans: in this case, one might (!) argue that simplification was more a reason for change rather than only a force)
- sociological and economical reasons: our social context changes (formerly tribal, newly industrial society: a huge difference which will result in language changes, no matter how hard you try to retain the 'old' language: social divisions form which didn't exist previously, you need to use new terms for new things in the real world, you mix with other peoples from other regions, etc): this, in my opinion, is one of the main
reasons for language change, but of course I am a sociolinguist and supposed to say something like that ;-) (my opinion would surely be challenged by structuralists and followers of the school of generative grammar: they most likely would accept that these factors are a force of change but might challenge for me saying they're the main reason)
- political reasons: yes, of course there was (and is) language planning, and of course it did affect the change of languages hugely, in one very popular case a dead language even was revived, successfully so (I'll let you guess ;-) and I don't mean Celtic Irish as one might dispute it's successfull revival); but again, this (for me) is only a force of change and not the main reason, because as I see the world, language politics is only an institutionalised form of sociological and economical change and as such part of the former point, although different from it
- language contacts: might be subsumed under sociological & economical reasons, surely they're important for language change, no one would dispute that, but again I would say it's only a force - not (necessarily) the reason, there are examples in history where two coexisting languages hardly did interfere one another until (until!) there was sociological change
- child speech: again, incorrect or incomplete language acquisition of language through children really is not the reason for change - on the one hand, language acquisition is not fully accomplished at the age of 15 (far from it!), on the other hand, children only talk differently from their parents when grown up if (and only if!) there was sociological/economical change, in my opinion (again, wouldn't go undisputed, I should add); sometimes, the parents themselves are responsible for children speaking differently because they'd prefer their children speeking a higher valued accent and so try to speak to them only in this accent, or alternatively send them to a school where they hope they'll learn an accent of the (higher) middle classes, as is the case in England and America (I think)
.
So again, this would more fit into sociological & economical reasons - as there are lots of examples where (in unchanging - or better:
very stable sociological conditions) language change did not occur (Islandic being a good example ...)
- language as social identity is of course a very important factor but again could be subsumed under sociological conditions: this is what sociolinguistics really is about ;-) - and this is not only about national and ethnic identity but about peer groups too
So, basically, in my opinion (with all the facettes which are to them), there are two main positions:
- language change is due to simplification in the generative grammar sense
- language change is due to sociological and economical change, this being the sociolinguistic position
Of course, this dispute under linguists is NOT about denying that sociological change does not play it's part or, on the other hand, simplification in the generative sense would not be important -
the dispute is one about establishing priorities - about what's most important, what's the reason behind change, what is the main force; and most likely this dispute never will be resolved (otherwise, what would be a linguists job if there weren't any riddles to solve any more

.
And the important thing to remember and to keep in mind, concerning language change, always will be:
there is more than .