I believe that a lot of countries fear China, and especially the possibility that China could become SEA's strongest superpower, these include Taiwan (obviously), Japan, South Korea, India and the USA. If these join togheter in some kind of a 'anti-Chinese alliance', then China will have a hard time. Especially Japan has started to become more aggressive towards China and friendlier towards Taiwan, these two joining in a U.S. backed alliance already could be quite a threat to China, and then we have India sitting at the 'backdoor'.
The Japanese Self-Defence Forces are also undergoing a quite rapid (and expensive) modernisation, and now with Koizumi re-elected they might drop the 'Self'-part and become 'ordinary' defence forces. Japan already has a navy that is both modern and guite large, a very god air force, and good fround forces.
Also: China has never forgotten the Japanese attack in the 30's and what the Japanese did then and in WWII, and Japan knows this very well.