Slavery in the Ottoman Empire

Why do people enslave one another? It seems that throughout history there is this reoccurring circumstance of people capturing each other forcing them to live a life of slavery. The psychological reasoning for this could be human’s innate desire to be above other groups of humans, but if we know this is wrong why does it continue literally from almost the beginning of civilization.
Slavery seems to be one of the basic steps in the beginning of most civilizations. The Ottoman Empire was no exception to this hypothesis. The difference though between slavery in the Ottoman Empire and the slavery that occurred in American history is how these people were captured. Originally, people who were enslaved were prisoners of war that were taken by the Muslim militaries. Interestingly, according to Islamic Law, slavery is not really frowned upon. It is encouraged to set slaves free but there is no real requirement or disapproval of it. The only thing that was strictly prohibited by Islamic Law was the enslavement of one’s fellow Muslim. Under absolutely no circumstance, not war, not rebellion, could one Muslim enslave another Muslim. In the Ottoman Empire, there was no one group of enslaved people but many. Whites, Ethiopians and Blacks were all used as slaves. As time went on, there were slaves being brought into the Empire from all areas of Europe and Africa. At this point, the type of slave in highest demand was women, preferable from Africa. These women had one main purpose and that was to become domestic help for the upper classes.
The slave system of the Ottoman Empire is very interesting. As an American it is hard to understand how slaves can be from more than one origin. Also the mere fact that slaves could actually build themselves up in society and obtain high ranking positions in the government is astonishing. Doesn’t that go against the entire basis of what slavery is?