What is the reason behind this kind of cruelty?

I met a girl who was a prostitute since 11 years old- forced into it by her parents. She got rescued now, at 18. She said that unlike the other prostitutes around she was more sensitive and she still had a soul and was really vulnerable. The customers loved her and they loved her because she started crying and begging very easily. The other girls were tough and didnt really care for anything. Those men werent cruel to them, not as much as they were to her. Why? What was their satisaction? One guy even made her belive her fell for her and cares for her...onl to laugh at her later and call her stupid for believing him. The used to beat her and call her names only to see her crying. What was their satisfaction?