We sometimes think of this admonition as primarily internal and emotional. Jesus' first audience heard it especially in terms of attitude and action, in terms of treating enemies as if they were friends, strangers as if they were kin, opponents as if they were family. That did not mean external action as distinct from internal attitude but simply placed emphasis on the former rather than the latter.John Dominic Crossan, The Essential Jesus (New York: HarperSanFrancisco, 1994, p. 163)
Think of it in terms of the rebels who brought doom upon Jerusalem. Considering Rome their enemy, they struck at Rome; which struck back, and destroyed the city, and changed Jewish history forever (in fact, some scholars see it as the end of Hebraism and the creation of Judaism). But what enemy did they hate? Caesar? Did Caesar even know they existed, except as a threat to the Pax Romana?
How do you hate an enemy you have no personal relationship with? What you hate, what you oppose, is an abstraction, not flesh and blood. When someone follows a celebrity around excessively, we don't say that person is in love, we say they are delusional. They love the idea of the celebrity, not the person herself. When we say we "hate" our political opponents, are we describing an external action, or an internal attitude? Can I act out my anger at a politician? Does the action have any meaning?
Conversely, can you love a politician? A person, perhaps, but an idea? What relationship do I have with that idea, and who is in charge of it? Me? or the idea? Can I love someone I do not know? How do I love without any personal relationship at all? If I say I love a celebrity, I am either a child, or a deluded person. If I say I hate them, I am placing myself at the center of existence, and insisting that only what I think is important.
The issue of center and boundaries and power and powerlessness are all of a piece, and all to be seen here. "Love your enemies"? That would require action, not attitude. I cannot love those whom I do not know. I can hate; but that is impotent, because hate is about power, about action, about exerting my influence over another. If love is patient, and kind, and endures all things, and hopes all things, and believes all things, then love is about powerlessness, and I cannot exert it over another, especially over one I do not know. Love is about the other, not about me. Hatred focusses on me, on my wants, my desires, my ability to gain power over another. Love focusses outward, but needs an object, needs a reciprocation, abhors a void. Loving an idea is trying to love the void. Hating an idea is simply trying to feed my ego, to convince myself of my own importance.
"Love your enemy." You first have to know who your "enemy" is; and who it cannot be.