Learning L1 vs learning L2

Why are the processes of learning first languages so different to learning second? 

Obviously, the brain is a bit different.  The child’s brain is built to act like a sponge for the purpose of taking in so much information in the first few years of a child’s life.  (the brain goes through a process of making connections called Synaptogenesis which is particularly prevalent during the “critical period” when children learn language).

But consider this, unless there is a physical disability, people learn languages.  According to the Universal Grammar theory (Chomsky 1965), a brain is not only designed with the capability of learning a language, it has been pre-programmed to do that.  A child’s brain just needs to be configured.

So what language do you want to learn?  One which puts the subject first? CHECK.  One that conjugates the verbs? CHECK. Do the verbs conjugate differently depending on the subject? Yes, CHECK.

Throw in some vocabulary and that is why children learn complete languages in the first 5 years of their life.  (On top of learning all that other stuff like who this "Mum" person is and what that hairy thing that eats from the bowl on the floor is).

But people need to be taught to write.  It seems that you cannot stop a person from naturally learning to speak but without the correct stimulation, children won’t learn to read.  Could just be because trying to read takes effort but your ears are always taking in input. Or it could be because speaking is just one of those things we do naturally like walking, we don’t need someone to teach us we are pre-programmed to do it.  But writing is a human invention.

What does this mean?  In my opinion, it means that if you are learning a language it is better to focus on the language first and not the method of recording the language. Focusing first on developing fluency and then moving onto writing to consolidate will ensure that you don’t become dependant on the written word or abuse your Monitor (see Krashen's Monitor Hypothesis).

http://en.wikipedia.org/wiki/Monitor_hypothesis