I remember having a discussion with my history teacher about this. I basically said the reasoning for the US getting involved in imperialism was, "If we don't get it, they will." They being the rest of the world. But a main reason the US took territory in the Pacific and Caribbean was because of the need for more resources. Another reason was how Americans wanted to spread their ideology around the world. At the time, all the powerful nations of the world controlled colonies and the US wanted to follow suit to be recognized as a world power so they took over territory.
