There has recently been a touch of disagreement on this site between Linda Jakobson of the Lowy Institute and Bonnie Glaser of CSIS about the motivations for China's actions in the South China Sea.
In short, Jakobson argues that China's decision-making can be explained by bureaucratic competition between China's various maritime agencies, whereas Glaser says it's the result of a deliberate, centrally organised policy of territorial expansion.