America's growing involvement in East Asia during this time was part of its overall move towards expansionism and imperialism. It was motivated by economics, military factors, and racial factors.
Much of America's involvement in East Asia came about because of a desire for more trade. This was why America "opened" Japan in the 1850s and it was why the US pushed the Open Door policy in China (asking for access to trade with China on the same basis as the European countries had).
The US also took the Philippines for economic reasons, but also for military and racial ones. It felt that the Filipinos needed to be "civilized" by superior Americans and it wanted the Philippines as a potential military outpost for projecting American power into Asia.