Good quote from today's "Foreign Policy" "When China Rules the Sea" by James Holmes.
China has yet to acclimate to the rules of the nautical game. Why? Because continental powers like China tend to think about the sea differently than natural seafaring states like the United States or Great Britain. Where nautical peoples see a commons — an ungoverned space, open for the free use of all — terrestrial peoples see national territory, to be governed as though it were dry land. A chasm separates Chinese from Western worldviews.
Could also be that the Chinese are just trying to assert themselves and intimidate. I don't know if China's naval strategy has to do with wanting purely to control the seas as national territory due to their being a terrestrial people. My own (amateur) opinion on China, navy-wise, consists of the following:
1) China was hugely embarrassed by the West (by the West's naval power) and then decimated by Imperial Japan
2) The modern Chinese government (I'd imagine) wants to make DAMNED sure that something like the Japanese invasion never happens again nor any embarrassments---China was actually embarrassed in modern times by the United States back in the 1990s when they were threatening Taiwan and then the U.S. moved in an aircraft carrier and scared them away
3) The Chinese also want to be a strong modern economy. Some say that historically China has been insular, but the thing is, even if China wishes to be insular today, that requires them to be a global player. They have to have secure access to trade routes, natural resources, etc...in order to have a functioning economy, and all of that requires having a strong, blue water-capable navy. So national security in the form of economic and military are both tied to having a strong navy.
4) China wants to be able to project power in that region of the world that it is in. And doing so in particular will require being able to check the United States