Nah man, I honestly think that hatred, just like love, is...How you say...Irrational? Unexplainable?
Of course, in recent times, I've disliked Juve more because I'm jealous & envious of their achievements. But historically, I hate Milan much much much more than Juve.
I don't believe there is no right or wrong, on who should be our #1 enemy. It all comes down to one's self.