The South China Sea is actually China's Caribbean, and not the East China Sea. By controlling it, China links the Western Pacific with the Indian Ocean and become one of the few continental great powers in human history to also be a maritime great power.
It's probably hard for peoples not in the immediate NE Asia area to understand the depth of loathing Chinese and Koreans have for the non-repentant Japan. What we're seeing today in NE Asia is another chapter in Sino-Japanese-Korean history over the last 1,500 years (or so), because the third Sino-Japanese war (1894-1895) never truly ended, and there was a lull from 1945 to a few years ago. And now, the interlude is ending and both sides are preparing for what comes next.The disagreement with Japan at this stage is merely an organ of this larger body of disagreement.