List the reasons why Africans enslaved each other.
First, consider the fact that Africa is, even today, tribal in nature, and that many of these tribes were enemies or at least rivals.
African slavery, though markedly less brutal than its European counterpart, occurred for a variety of reasons. Status within the tribe in terms of wealth and power, profit in terms of dealing with Spanish and Portuguese slave traders, and then as an informal type of warfare, kidnapping your enemies as slaves was considered the equivalent of "counting coup" in Native American culture.
See the diary of Olaudah Equiano for some descriptions of his experience in African slavery, and later on the Middle Passage.
Many African tribes have had slavery as part of their society for thousands of years. Many of the African tribes are warring tribes and when they would capture their enemies they would enslave them. Also there are tribes where there is a hierarchy system that includes slavery, so it is a position that people can be born into. Actually when the Europeans came to Africa to first enslave them, they used warring tribes against each other to help capture Africans. They would be bribed with guns, and metals and then help the foreigners by capturing tribesmen from an enemy tribe.
The practice of slavery is known to have existed for many thousands of years in all parts of the world, not just in Africa. Practice of slavery in one form or other has been used for thousands of years by those in power to exploit others not so fortunate. It will not be an exaggeration to state that slavery is older than even the humanity when we consider that even some species of insects raid and enslave completes colonies of other types of insects. We must also realize that domestication of animals and keeping animals in cages is nothing but enslaving them.
The reasons of all practicing slavery by Africans is same the ones applicable to any other people for practicing slavery or for any other form of exploitation. This reason is desire for gaining personal comforts, wealth, and prestige, without regard or compassion for others. It is worthwhile noting that till about nineteenth century, all people around the world lived at near subsistence level. Without the help of modern technology, in those time only way one could acquire personal wealth and special comforts, only by exploiting others. Even in earlier days of industrialization, the industrialists exploited the labour to such an extent that the condition of these labour was often worse than those of the slaves in earlier periods.