Because without finding it in yourself, you’ll have a hard time recognizing it in others.
In a recent blog post, Vox Day concluded that “[SJWs] understand your love as poorly as you understand their hate.” The lay observer who is not in touch with that emotional state might very well see the massive financial investment in a movie franchise (like Indiana Jones) that treats the foundational story with such casual disdain and conclude that they’re just bad at their job. Au contraire, they’re actually quite good at what they do, as one can readily tell by their cultural success over the past 50 years. To conclude ineptness is a mis-read of their intentions, born from projection and a failure of empathy… which is itself a byproduct of a void where the moral virtue of hatred (as a capacity) ought to be. You just can’t see it if you aren’t familiar with what you ought to be looking for.
When it takes one to know one, perhaps the greatest value in developing one’s capacity for hatred is the development of one’s ability to detect it in others.
Mandatory plug: the book is (somehow) still available here.