First of all, it's important to explain what ISOLATIONISM is. ISOLATIONISM meant that the United States of America would remain isolated or that they would not interfere with other countries' businesses.
By the late 1800's the United States realized they had to change this policy called ISOLATIONISM and they had to start expanding. There were specific reasons for expansion that had to do with the military, the economy and with nationalism.
Economically speaking, the United States had to expand in order to start selling their products in different countries. American industries needed to be positioned in foreign lands.
As for the military reasons, the United States had to match European naval force in order to protect the country and their overseas businesses.
Finally, nationalism demanded the United States to spread civilization to uncivilized lands and territories throughout the world. This came from the idea of "the white man's burden", asking Anglo-Saxon men (Britons and Americans) to spread their superior morality and intellect upon the uncivilized nations and people of the world.