Here is an interesting article comparing people and sunflowers.  Just as sunflowers turn towards the light, so do people.  They will tend towards the light, whether that is through telling a girl how pretty she is (and she'll want to dress and groom in that way) to telling a boy who is an athlete how good he is (and he will practice and do things to get more similar encouragement).

This could be looked at negatively (as this article kind of does) and positively.  Treat someone with respect and they will respond.