I think that a driving factor in the search for new markets was government's hand being forced by business interests. Essentially, at the point of imperialism, American businesses were afraid that the market was contracting, or "drying out." The desire to expand markets was the rationale behind the drive for foreign markets and new revenue streams for abroad. It is a strong example of how business interests influence policy making decisions. This close relationship between both business and government ensured that new markets would always be available through foreign policy initiatives.
I assume that you are talking about what happened in the later parts of the 1800s, when the US started to do a bit of imperialism, taking stuff in the Caribbean and the Pacific.
If so, here are factors that helped cause this -- I think there are two of them:
- European countries were taking as much territory as they could and the US would have wanted to get some before it was all gone.
- The US was in the middle of a big boom in industry. As US companies came to be able to make more stuff, they wanted to have more places to sell the stuff they made.
Beginnings of American Imperialism:
The following information will not exactly answer your question, but it will give you information which will help you compose your answer.
Southern observers foresaw before the American Civil War that northern industrialists desired to absorb Central America and the Carribean. Southern observers said that northern industrialists intended first to exclude slavery from the territories of the United States so that states could be established whose U.S. Senators would not oppose the tariffs and improvements of harbors and construction of railroads at taxpayers’ expense for the industrialists’ benefit. When they had a Constitutional majority (3/4 of the whole) of states under the control of pro-industrialist governments, they would pass a Constitutional amendment to abolish slavery in the states. Abolishing slavery would bankrupt the southern plantation owners. Northern industrialists would buy the plantations dirt-cheap, establish peonage to work them, and export the excess former slaves to northern factories for cheap industrial labor. They would then absorb Mexico, Central America, and the Carribean and establish peonage in all of these areas to raise coffee, cotton, tobacco, and sugar, for supply of their northern factories.
An article describing this process was published in DeBow’s Review issue of February 1857.
What actually happened was little different than this prediction. Slavery was forbidden in the territories; a Constitutional amendment was made to abolish slavery in the states; cheap labor in the form of ex-slaves and cheap land in the South became available to northern industrial interests; the new industrial America partly absorbed and partly dominated the Carribean; Central America was dominated without being absorbed; the Philippines, Hawaii, and several central Pacific islands were absorbed; a big hunk of China was dominated. All, that had been predicted, took place, and more.