The artificial islands are in response to the US holding the first island chain [1]. Without a foothold in the South China Sea, China's military and economic access to the outside world is at the mercy of the US. This is the same reason you see so much Chinese interest in Burma — unbridled Indian Ocean ports [2].
1. https://en.wikipedia.org/wiki/First_island_chain
2. https://en.wikipedia.org/wiki/String_of_Pearls_(Indian_Ocean...