The US is a maritime power. Its foreign policy is closely tied to controlling the international shipping lanes. This is the reason why they are trying to dominate the bottlenecks, like SCS and Persian Gulf.
The US wants to use India to contain PRC. India wants to use the US to contain PRC. PRC wants to use Pakistan, Maoists, North East militants to contain India. India wants to use Japan and Vietnam to counter PRC. However, the US' goal is to have its own control, and not to help any country control these seas.
This is a big game of cat and mouse.