Closing of the frontier--years of expansion and the ideas of Manifest Destiny had hit a wall with no where else to expand. Combined with other factors, imperialism offered a new place for the US to expand their culture and ideas.
Industrialization--industrial growth demanded markets and resources. Imperialism offered new places to sell industrial goods as well as an opportunity to get access to natural resources.
Need for navy growth--Alfred Mahan wrote a series of books arguing the need for a US navy in particular ports. With trade contacts in Asia, Mahan argued the US needed island nations to place naval ports. Many islands were acquired through imperialism which helped the US to create their goal of naval ports.