Because God is love, and because Jesus Christ is pure love, therefore he can give us no other commandment than this: that we should believe in the name of his Son Jesus Christ and love one another (3.23). God wants us to become like he is; Jesus Christ wishes to transform us into his own glorified state (3.2), which means we must purify ourselves of everything that stands in the way of our living in true love (3.3).
The measure of our relationship with God is our belief in Jesus Christ and our love for others: Whoever does not love does not know God, for God is love (4.8). It doesn't matter what you believe; it doesn't matter what doctrinal statements you affirm wholeheartedly; it doesn't matter what confession you grace with your loyalty. If you do not live in love, the true revealed love of God in the self-sacrifice of Jesus Christ for the sake of sinners and for the whole world (2.2), then you do not know God. If you have hatred in your heart, if you are angry with others and this anger defines your life and your thinking, if you don't wish well for those around you who are suffering, then you do not know God. He is still a stranger, as far as you are concerned.
God is Love, and so anyone who knows God must also love others. Jesus Christ, who alone knew the Father (Mt 11.27, Luke 10.22), reveals to us the great depth of his love through his death for all the sinners in the world. This the very essence and core of the Christian message, and those who meditate and who are transformed by this message, this truth, this reality will find themselves filled with the same love which God has for all and every.