Jesus appeals to us to increase our capacity. Increasing our capacity is hard work and uncomfortable. Yet, most of our faith journey is lived in a place of discomfort. To sound, live, or act too much like our current culture is to be rather removed from God’s ways and voice. When we consider Jesus’ first audience were individuals steeped in the Jewish law, Jesus telling them to love their enemies was a call to increase capacity to hold differences. The first-century Jews lived by over 600 laws (contained in the Hebrew Bible/Old Testament), had permission to take an eye for an eye, even a life for a life. Enemies did not have to be loved. You loved yourself as your neighbor and you were good (Lev 19:18). That covered love for over a thousand years, until Jesus puts forth an invitation to his followers and us to expand our capacity and love enemies. Imagine, Jesus asking liberals to love conservatives and conservatives to love liberals. Jesus is asking you to put down your defensiveness and open your heart and listen to the “other side.”
When my father was preaching and his words turned to practical application he would smile and say, “I’ve stopped preaching, and I’ve taken to meddling!” Sorry!
How does increasing capacity happen? Slowly. Intentionally. Painfully.
Might I suggest capacity is more membrane-like in its ability to accommodate as we, in love, respect, and kindness, expose our hearts and minds to a variety of ways of living. However, this is an uncomfortable path and the way to deepening ourselves in God’s love. If you desire to increase your capacity to love, be of service, and live in the freedom of God’s spirit. Begin by noticing where you are defensive. The need to defend is generally (re-read the word: generally) from the false self. The true self does not need to defend, the true self can actively listen and not need to correct, advise, or fix the listening. It will take time. It will take you to sitting in stillness with God. It will take you to a view of yourself that might be unpleasant and yet honest. It will take you where you may not want to go. 
Come increase your capacity to love those you do not care to love.
 Jesus told Peter that he would be lead where he did not want to go as he aged and grew spiritually (John 21:18). This encouragement fits for life today.