Thats because most of us are born of the flesh. Minds of a person born of flesh are always evil. Forgiveness does not come in such mind because Satan plays a major role in such a person. They are Satan's disiciples and will do everything to please him. Whereas a man born of the Spirit will always humble himself and do what is pleasing to God. For such person, forgiving someone is very simple. When we love are enemies, the Father in Heaven will pour His abundant love upon us. On the context, we defeat Satan.