Generally speaking, Japanese think they are the superior Asian race. From that perspective Japanese aren't all that "nice" to Chinese and Koreans. They also feel inferior to the caucasian races, which may be interpretated as being "nice" to Americans and Europeans etc.
At this time Many Japanese companies see a closer relationship with China (joint ventures) as being crucial for business survival. So just now They are getting a lot "nicer" to the Chinese.
Japan's younger generation seems to be moving away from this pattern and appear to be more resolute about taking their place in Asia, and do not view the USA as the "great white hope."