Let's take a look at the Scramble for Africa. If we look at imperialism in Africa, we can identify nine major motivations or reasons for colonization:
New sources for raw materials
New markets for finished goods
Nationalism, or the expansion of European culture
Missionary activity, or the expansion of Christianity
Strategic military and naval bases
A place for excess population to migrate
Social and economic opportunities
The "White Man's Burden" (today regarded as thinly veiled racism)
Let's explore them in more detail.
1 - Sources for raw materials. Initially, Spain, France, England and Portugal set up mercantile colonies in the "New World" for this reason. It was a source of new raw materials and natural resources, which could be harvested by implementing a labor system (African or native American slavery, the encomienda tribute system, the mita labor lottery, etc.). Thanks to explorers like David Livingstone traveling through the heart of Africa and reporting on his findings, Europeans turned their attention to Africa. Some of the most attractive natural resources in Africa were diamonds, gold, and rubber (concentrated mostly in the DRC, or as it was known during the imperial period, the Congo).
2 - New markets for finished goods. Long the industrial powerhouse of the world, Europeans would import raw materials and natural resources from their colonies, process and manufacture them, and resell the finished goods back to settlements in the colonies.
3 - Nationalism. As European countries fought for more land (because land was money), they scrambled for new colonies in Africa. Part of this was because of the desire to share in global profits, but another reason was nationalistic: colonizers took pride in their country and wanted it to be a global leader.
4 - Missionary activity. Northern Africa had been Islamic since the Umayyads, African Kings and nomadic Berber peoples of Northern Africa, adopted and spread the religion beginning in the eighth and ninth centuries. Europeans had always hoped to find Christian kingdoms in Africa, which is what prompted some Portuguese explorers in the fifteenth century to make so many stops in Africa on their way to the Spice Islands. New colonies in Africa would not only make Europeans richer but also help spread Christianity.
5 - Strategic naval and military bases are key to some countries. England, for instance, took control of the Cape of Good Hope (the southern tip of Africa), and after fighting the Ottomans, the Suez Canal. Both of these were key to controlling trade between Europe and the Indian Ocean. By controlling the southern tip, the British navy could control ships traveling around Africa, and by controlling the Suez Canal, the British could control ships hoping to cut directly from the Mediterranean Sea into the Red Sea and Indian Ocean. For the British, controlling trade was about dominating the global economy.
6 - Excess population migration. While not one of the main motivations for colonization, this does account for some immigration into African settlements, which ties into the next motivation:
7 - Social and economic opportunities. Much like the original settlers who came to the new British colonies in North America, some traveled to Africa to make new economic opportunities out of the wealth in natural resources and raw materials.
8 and 9 - Humanitarian reasons and “The White Man's Burden.” The phrase “white man's burden” comes from Rudyard Kipling's poem of the same name (see reference below), which essentially offered these two motivations for colonization. Some Europeans felt that the African states and cultures were not as civilized as the Western world because they did not possess the technology that their northern counterparts had. (Note: Author Jared Diamond argues that this phenomenon had nothing to do with any ethnic group being inferior; it occurred instead because the natural resources available in Africa differed markedly from those available in Europe. For more about this theory, read Guns, Germs and Steel). In order to "civilize" the Africans, some Europeans felt it was their duty, the "White Man's Burden," to colonize and westernize the Africans.
As far as reasons against imperialism in Africa, David Livingstone, the famous English explorer who inadvertently inspired European nations to colonize Africa, was vehemently against the idea of colonization. He valued African culture and its landmarks, and when approached by American journalist Henry Stanley to help set up imperial colonies, Livingstone refused to help. Stanley later went to King Leopold II of Belgium, who agreed with Stanley, and Leopold set up the most brutal colony of all in modern-day Democratic Republic of Congo.
Generally, there were few Europeans advocating arguments against imperialism in Africa.