In a way, pretty faces are rewarding. They make us more likely to think good thoughts. There are some underlying processes going on in the brain that prejudice us to respond to attractive people better even if we are not aware of it.
Faces hold a special power for us, perhaps more so than art or objects. The beauty bias has a real influence upon us, something we should be mindful of when dealing with others.
There are no definite rules to what kind of face can be called beautiful, but we chose faces of extreme -- very ugly or very pretty.