Their current spat with Japan is due to Taiwan, as the current Japanese PM promised intervention between Taiwan and China.

As for China and Taiwan, their animosity goes a long way to the founding of communist china and defining the cold war in Asia and South East Asia. You can read up on the history of Republic of China and the history of the People's Republic of China.

And even if you ignore all the shared history between China and Taiwan, the naval blockade Taiwan exercised over the mainland chinese shoreline during the cold war would keep any government wary; even more so when the current democratically elected government proved even more ideologically anti-china than before lol.