The correct answer to this open question is the following.
Industrialization transformed American life in the late 19th century as nothing had before. This industrial change transformed the life of the American people. Technology changed farming and many farmers decided to move from the rural areas of the United States to the larger cities where the large factories where located. Those new fabrics were offering many jobs to operate the new machines. Although those were low paid jobs, they represented a relief to the difficult conditions on many Americans.
Important businessmen such as Andrew Carnegie and John D. Rockefeller created huge companies in the steel industry and oil industry, respectively. Although they invested a lot of money to create successful companies, during the Gilded Age those companies were accused of monopolistic practices.
The Articles of Confederation gave the states way too much power and the federal government not enough power. The Constitution gave the federal government more power an reduced the power of the states.
Economic forces such as survival of the fittest, political forces such as Britain's "place in the sun", and social forces such as "the white mans burden" were most responsible for the new imperialism that began in the late nineteenth and twentieth centuries.