Here's the best way I know of. It's really complicated, and hard to accomplish. You ready?
Keep their luxury higher than they need.
Simple as that. Get them to max obedience, and keep them in enough luxury that they don't complain. If you do that, they will slowly come to like you. I don't know if more luxury means faster love, but that's the best way I've been able to find. I think there's a spell that increases affection, but I can't remember off the top of my head. And, finally, you can temporarily boost their affection a bit by periodically, not too often, about once a week or so, giving them praise/a present.
It's not too hard, really. But if they ever start complaining about living conditions/quality of life, you need to add more luxury. As long as they're above that line, they'll slowly begin to love you.