First of all, I think the Chinese attitude right now is more or less that possession is nine-tenths of the law. Secondly, China has now proclaimed itself to be a maritime power. It is the world's largest exporter. It basically sees the South China Sea as its home waters as a maritime great power.
From the Chinese point of view, this is not an expansion: it is historical. We can take that seriously in the sense that, on the one hand, China is in some ways flouting UNCLOS on this. On the other hand, the nine-dashed line should be seen as a limit in terms of territorial disputes.
China certainly does have, and will have over the years, a further desire to be a maritime great power. Basically, the Chinese look at the world map and see that all the islands of the world in the oceans are now occupied by what were 19th century imperial powers: Britain, France, the United States in some cases, and a residual few for other colonial powers. China says that's simply not fair, that it is now a maritime power, and that the one place it can claim is this one.
In some sense, that's the explanation of the South China Sea and “why now”, but we should be aware of the fact that China does intend to be a maritime great power. It now has a base in Djibouti on the west coast of Africa. It is the first overseas Chinese base in history. I think that as the world's number one exporter, with the largest commercial fleet in the world, China is here to stay.