You can forgive and do good to your enemies, pray for thy neighbor, or help strangers, but what is it to love them? Perhaps Christians think "X loves Y" is a formula such that you can substitute "X" and "Y" with anyone and what relates them can still be love. But love isn't like that. Whether you can love someone depends, among other things, on how you see them or what you understand them to be. If you understand someone to be your enemy, or just your neighbor, or a total stranger, can you still love them? Maybe you can, but this is at least not a straightforward question that has a clear answer.
That's why I saw a kindred spirit in Rush Rhees when I was reading these remarks by him: "The way in which Christians speak of love seems to me one of their most perplexing and (for me) one of the most discouraging sides of their teaching." ("Christianity and Growth of Understanding", in Rush Rhees on Religion and Philosophy, edited by D. Z. Phillips, Cambridge University Press, 1997). In the same essay Rhees also points out another problematic aspect of "Christian love", namely, the power it is supposed to have:
In the Gospel the story of the good Samaritan seems to be intended to give the sense of 'Thou shalt love thy neighbour as thyself'; and this commandment ... or am I wrong? I was going to say that Christians seem to look on this commandment as a rule which, if only it were followed, would solve all the difficulties in human relations. And when I think of it in that crude form, my jaw drops.
Rhees thinks this "commandment", in "that crude form", does not tell you "much about your relations with people you have to live with day by day". And it is, I would like to add, through living with these people that you acquire a robust sense of what it is to love someone.