I love The Message's interpretation of this passage.

Matthew 5:43-48:
You're familiar with the old written law, 'Love your friend,' and its unwritten companion, 'Hate your enemy.' I'm challenging that. I'm telling you to love your enemies. Let them bring out the best in you, not the worst. When someone gives you a hard time, respond with the energies of prayer, for then you are working out of your true selves, your God-created selves. This is what God does. He gives his best—the sun to warm and the rain to nourish—to everyone, regardless: the good and bad, the nice and nasty. If all you do is love the lovable, do you expect a bonus? Anybody can do that. If you simply say hello to those who greet you, do you expect a medal? Any run-of-the-mill sinner does that.

In a word, what I'm saying is, Grow up. You're kingdom subjects. Now live like it. Live out your God-created identity. Live generously and graciously toward others, the way God lives toward you.