The sociopath wants the person to be easily enough fooled to stick with him. This can be accomplished by looking for someone who is very, very loyal. Most of us consider loyalty to be a very positive trait - and it is a positive trait. But it also blinds people to some of the traits of the person they're loyal to.
Looking back now on the whole sexual scene we can see that our species has remained much more loyal to its basic biological urges than we might at first imagine. Its primate sexual system with carnivore modifications has survived all the fantastic technological advances remarkably well.