Because hatred is a passion of the sensitive appetite, which is moved by an apprehension in the senses.
 Thomas Aquinas, Summa Theologica (1274). copy citation

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Author Thomas Aquinas
Source Summa Theologica
Topic passion appetite
Date 1274
Language English
Reference
Note Translated by Fathers of the English Dominican Province
Weblink http://www.gutenberg.org/cache/epub/17897/pg17897-images.html

Context

“The reason why we love those who are straightforward is that they make known the truth, and the knowledge of the truth, considered in itself, is a desirable thing. ________________________ SIXTH ARTICLE [I-II, Q. 29, Art. 6] Whether Anything Can Be an Object of Universal Hatred? Objection 1: It would seem that a thing cannot be an object of universal hatred. Because hatred is a passion of the sensitive appetite, which is moved by an apprehension in the senses. But the senses cannot apprehend the universal. Therefore a thing cannot be an object of universal hatred. Obj. 2: Further, hatred is caused by disagreement; and where there is disagreement, there is nothing in common.” source