ASEAN is made of states with clear boundaries. If you look at the whole of ASEAN, the only place where the boundary is not clear is the South China Sea.
ASEAN as a single player doesn't exist; it's always a collective voice. ASEAN is that kind of creature. You cannot have eight members saying one thing and two others saying no.
China wants to deal with the South China Sea issue on a bilateral level. Because, as some Chinese experts have emphasized, it's a sovereignty issue and you don't want to bring in a third person, for a third people will make both sides unhappy. That's the logic.
China is now a big power, why should it ask a third person to decide its own issue? Hopefully, other dimensions have become more important such as trade and people-to-people communications, perhaps in the end those clear boundaries don't matter that much.
Asia is witnessing the re-rise of China, as in history China has always been a powerful country. It's difficult to handle someone who is much stronger than you, so a lot depends on how China sees itself. A Western view is that China will rise, expand and absorb everything, but I don't think that's how the Chinese think.
Because China has been so successful in the last 20 years, we forget how quickly that happened. The Deng Xiaoping model is to build up China using international trade as the logic for how China's institutions will change and adapt to the modern world. China has worked in this way so far, and will continue doing that. China's transformation from the old system is still going on.