It all comes down to the fact that the South Eastern Continental Shelf is awash with gas and oil. The more territory that you can claim, by fair means or foul, means that you get a bigger portion of it. China needs lots of it and it is now in the position where it can shoulder aside other countries to get it. You cannot blame them. The islands in dispute have had minimal, if any, development in recent histrory so as far as they are concerned they can go back a thousand years, or even more, for written confirmation from some Chinese admiral who landed and claimed it for the Emperor.
How far back do the maritime laws of possession go? The Chinese can go back 3,000 years.