Know and rely on the love God has for us. What do we rely on?  Cain loved himself and brought stuff to God, but God didn’t want his stuff, he wanted Cain.  Cain did not rely on the love God had for him.  Able brought the best of what he had to God, he brought himself and relied on the love of God.
See how great a love the Father has bestowed on us, that we would be called children of God; and such we are. For this reason the world does not know us, because it did not know Him. For this is the message which you have heard from the beginning, that we should love one another; not as Cain, who was of the evil one and slew his brother. And for what reason did he slay him? Because his deeds were evil, and his brother’s were righteous. (1 John 3:1, 11, 12 NASB)
There is no logical reason to obey the commands of God except that God loves us. If we have trouble loving our brothers and sisters it’s because we are having trouble understanding the love God has for us.
Dear friends, let us love one another, for love comes from God. Everyone who loves has been born of God and knows God. Whoever does not love does not know God, because God is love. This is how God showed his love among us: He sent his one and only Son into the world that we might live through him. This is love: not that we loved God, but that he loved us and sent his Son as an atoning sacrifice for our sins. Dear friends, since God so loved us, we also ought to love one another. (1 John 4:7-11 NIV)