To make a strong argument, the writer or speaker must avoid logical fallacies.
This is because a fallacy is something that is untrue, thus if it was logical your reader might be persuaded against your argument. Basically, as a writer you don't want to contradict yourself and you only want to support one side of an argument.
Answer: Romeo describes her as light because he has new found love within her that he feels brought him out of the darkness and into a whole new world of emotions and feelings.
