Taming slaves requires their different stats such as trust, love, and obedience to be high. After a while, they will 'submit' to you and they are then tamed. Keep training and rewarding them and their levels will increase.
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If you want to increase affection you would have to send them out to prostitute themself then catch up with them to build that up.
Because the NPC interaction system and the NPC slavery system don't really interact.
Or are you wanting to increase your slaves love for you? Then you just need to reassure them.