In a word: power. Imperialism is the process by which one country dominates another directly, by political means, or indirectly, by economic means.
As we will discuss in the following sections, imperialism has changed over time, particularly during the last one hundred years (where its forms and methods have evolved with the evolving needs of capitalism). But even in the classic days of empire building imperialism was driven by economic forces. In order to make one's state secure, it had to be based on a strong economy; and by increasing the area controlled by the state, one increased the wealth available. Therefore states, by their nature, are expansionist bodies, with those who run them always wanting to increase the range of their power and influence. This can be best seen from the massive number of wars that have occurred in Europe over the last 500 years, as nation-states were created by Kings declaring lands to be their private property.
Here we will focus mainly on modern capitalist imperialism. As power depends on profits within capitalism, this means that modern imperialism is caused more by profit and other economic factors than purely political considerations (although, obviously, this factor does play a role). As will be seen in section D.5.1, imperialism serves capital by increasing the pool of profits available for the imperialistic country in the world market. This is the economic base for imperialism, allowing the import of cheaper raw materials and goods and the export of capital from capital-rich areas to capital-poor areas (in order to benefit from lower wages and fewer environmental and social controls and laws). Both allow profits to be gathered at the expense of the oppressed nation. In addition, having an empire means that products produced cheaply at home can be easily dumped into foreign markets with less developed industry, undercutting locally produced goods and consequently destroying the local economy along with the society and culture based on it. Empire building is a good way of creating privileged markets for one's goods.
Since capitalism, by its very nature, is growth-based, it must expand in order to survive. Hence capitalism is inevitably imperialistic. In pre-capitalist societies, there is often extensive cultural resistance to the attempts of foreign capitalists to promote the growth of the free market. However, "primitive" people's desire to be "left alone" was rarely respected, and "civilisation" was forced upon them "for their own good." As Kropotkin realised, "force is necessary to continually bring new 'uncivilised nations' under the same conditions [of wage labour]" [Anarchism and Anarchist Communism, p. 53]