Jesus is not an unrealistic sentimentalist. He acknowledges that we have enemies. He simply says that we should love them. Why? Because God does ... and we are now members of God's family, called to have God's traits.
To be a member of God's family is to live according to God's values. It is to bring the kingdom of god closer and closer. That's why Christianity, at its roots, is not simply a system of doctrines. It is a new way of life. When we go into the baptismal waters, we "die" to one way of life and rise to a new way of living.
You are to look like, act like Jesus. And the way you treat your enemies is a good measure of whether you've caught on to this.
You should bring to mind some of your enemies. Then you should turn to the Lord, and ask, "How do you relate to these people?"
Then the Lord and you need to have a talk.
- Little Black Book, Diocese of Saginaw