We love, because he first loved us. If any one says, "I love God," and hates his brother, he is a liar; for he who does not love his brother whom he has seen, cannot love God whom he has not seen. And this commandment we have from him, that he who loves God should love his brother also.
1 John 4:19–21 (RSV)
We often mistake self-satisfaction for love. It may be a dim vision of the love St. John holds out for us to see, but it is not close to the love of God. We measure 'love' by what we gain from others. We expect more to flow toward us than we give out.
Our miserly excuse for love is not far from hate, for it turns if we don't get what we think we deserve in return. We offer the Father whatever feelings we can generate within ourselves and call that love. Once we have done that, there is rarely any left.
When St. John teaches that if we do not love our brother/sister, we do not love God, the calculations begin. We compute the limits of love in our hearts, just enough to convince ourselves we love God as well.
The Father shows the measure of love in the cross. There His Son suffered and died at the hands of the loveless. His love bursts forth from the tomb, redeeming all creation. The infinite cosmos is far too small to contain this love.
Our starting point is the cross. We are baptized into Jesus' death for the sins of the world. It continues in His resurrection as the new creation replaces our old sinful nature. The Holy Spirit pours the Father's love into our hearts so we by faith may begin to love as we have been loved.
In the love of the Father given to us by the Holy Spirit, we live in the self-giving love of our Lord Jesus. It will never be a perfect love within us, but it will be the love that has loved us. We will learn to love our brothers/sisters in His love.