The Chasan Sofer explains, the yetzer hora is like a salesman trying to sell fake goods like the wife of Potifar tried to convince Josef that she was for his benefit, but when she nudge him day by day, he did not listen to him because if the merchandise is really good for him, she should not need to talk so much. The yetzer tov does not talk much because he is selling good stuff and the merchandise speaks for him. So the yetzer hora must have an advantage, otherwise he will not be listened to.
