As China has continued to grow in economic power and military confidence, it has slowly been testing its neighbors and expanding its area of influence. The South China Sea has multiple claimants who all argue that their traditional, historic boundaries offer them some sovereignty over the waters. China, in particular, claims almost all of the entire area in question. When exactly did it have control over this particular area? Which country could actually claim to have "first dibs" on the sea?
The International Law of the Sea is supposed to regulate who owns the sea. To summarise, nobody can claim ownership of the sea itself - all ownership is based off control of adjacent land. This is why control of tiny, isolated rocks can become important - not because the rocks themselves contain anything of significance, but because they bring with them control of the sea.
China doesn't actually claim the South China Sea directly - they claim all of the Spratley Islands, which, if it were true, would give them an EEZ covering pretty much the whole South China Sea. (Note that an EEZ isn't exactly the same as ownership, but it would give China control over any maritime resources in the area, which it's been speculated might be substantial... although they also might not be).