For a detailed look at exactly why people learn languages check out this longish article from zompist.com. To the point, jargon free and not afraid to poke at sacred cows, for example the myth that children learn languages easily:
This is a popular commonplace, and one asserted by linguists as well, mostly due to Noam Chomsky's belief in an innate 'language organ'. (Steven Pinker's The Language Instinct popularizes Chomsky's ideas.) Unfortunately, the evidence is against it.
Children begin learning languages at birth (infants pay attention to their parents' voices, as opposed to random noises or even other languages), and haven't really mastered its subtleties before the age of ten years. Indeed, we never really stop learning our language. (See David Singleton, Language Acquisition: The Age Factor, p. 56.) This isn't exactly the sort of behavior (like foals walking an hour after birth) that we call 'instinct' in animals.
But at least it's effortless, isn't it? Well, no, as we can see when children have a choice of languages to learn. What's found is that, to be frank, children don't learn a language if they can get away with not learning it.
When you realise how long it takes Japanese kids to learn their own language it puts your own language learning into perspective.