There were several reasons why expansion seemed like an attractive choice to the United States. One reason could be for protection. Some Americans wanted to acquire the territory of Canada in order to protect themselves from a British invasion.
Another reason could be interest in natural resources. This is common in territories that have resources such as lumber, oil and coal.
Finally, another reason for expansion could be an interest in establishing a sphere of influence in a different region. For example, the United States might want to acquire a territory in Asia, or Latin America, in order to be able to exert some influence in the surrounding countries.
