Let's start with why, more or less, American imperialism took hold in the first place.
Americans were motivated to imperialize during the Gilded Age because American industry had grown large and needed new sources of raw materials and overseas markets to sell goods, the American frontier in the West was closed and there were no new lands to expand to, and Americans felt the need to keep up with other European imperial nations who were building colonies. During the Gilded Age, America began to think of itself as an equal power to European nations.
So, the U.S. built a modern navy to compete with European militaries, Social Darwinists sought to "civilize" the "inferior races" by spreading technology, Christianity, and democracy, the U.S. declared an Open Door Policy in China to allow free trade, Teddy Roosevelt used Big Stick Diplomacy to develop an active foreign policy with a strong navy to accomplish goals, and TR added the Roosevelt Corollary to the Monroe Doctrine, giving the USA police powers to protect Latin America from European Imperialism.
Essentially, by the 20th century, the U.S. was a world power. The U.S. had transformed into an economic power, it had a large navy, it had annexed territories in the Caribbean and Asia, and used its influence to build the Panama Canal, protect Latin America, and trade in Asia.
When WWI broke out in Europe, Wilson tried to keep America out of the war. However, Americans were outraged by German violations of U.S. neutrality and free trade as well as the sinking of passenger and cargo ships. American imperialism swelled American's sense of national pride and the time had come to demonstrate America's might by entering the war.