I see it as a challenge to empathies with your enemy even more than you do with your friends.
One needs a very strong ego in order to understand his enemy emotionally, and still be his adversary. It requires one to have an inner strength (the power to create) that is completely independent of reactive antagonism.
The people who cannot (note: not "would-not" but "can-not") understand their enemies, are the ones who project their own weakness on to others in result of self-denial (even a denial of weakness is a weakness).
When you ask such persons "who are you" they will likely say "I'll tell you what I'm not
They are the ones most prone to "snap" quickly and be aggressive (… the best defense is offense).
It's all about that inner strength to define one's self independently.
(It's also true in the macrocosms- Israel, for example, is very much based on "against" rather than on "pro"- which is why the leadership is crumbling down so horrifically. But I digress.)That is not to say that one shouldn't have enemies- it's even vital to have them…
but one should be aware if one's animosity is based on neuroses or on a realistic threat.
I'm somewhat doubtful that's what Sun-tzu meant, but I'm pretty sure that's what Nietzsche meant (LightAngel beat me to the quote).
So I'm not shocked anymore if some-one turns me down...because that's what I expect from the gross of humanity anyway.
It's rational not to expect much from human beings, however one should be careful of self-fulfilling prophesies.