Why was slavery seen as a good idea by some people in 1700 and 1800?

It was because the people of Africa were strong and beautiful. The traders saw them and thought they would be better slaves and easy labor to get. Some also believed that they were giving the black slaves a better life in a colonized world and giving them religion and a chance and "life". It is also said that some of the slave owners quoted, "Once a black man, always a black man. You can never lose a black slave."