In plain words
This is man's chief duty. So we must not hurt anyone's heart. The reason is that we will have to suffer that hurt ten times over.
Good and bad both exist in this world. As the feeling, so the result. So when we first purify our feelings, our life will change. So become full of Prema. The Prema of God is very sacred. If we earn even an atom of that one Prema, we will experience unbroken Ānanda. Develop Prema in your hearts.