Economic factors. The US wanted to take an empire to give it new markets in which to sell goods and from which to buy raw materials.
Military factors. This was the time of Alfred Thayer Mahan’s ideas about the dominant role of naval power. The US felt that taking an empire would give it more naval bases from which to project power. This would allow the US to become a more important power.
Cultural factors. Many Americans believed in the idea of the “white man’s burden.” They felt that they needed to go out and help to civilize the rest of the world.