I answer that, As stated above (A. 1) , love consists in a certain agreement of the lover with the object loved, while hatred consists in a certain disagreement or dissonance.
 Thomas Aquinas, Summa Theologica (1274). copy citation

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Author Thomas Aquinas
Source Summa Theologica
Topic love agreement
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

“Therefore love is not the cause of hatred.
On the contrary, Augustine says (De Civ. Dei xiv, 7, 9) that all emotions are caused by love. Therefore hatred also, since it is an emotion of the soul, is caused by love.
I answer that, As stated above (A. 1) , love consists in a certain agreement of the lover with the object loved, while hatred consists in a certain disagreement or dissonance. Now we should consider in each thing, what agrees with it, before that which disagrees: since a thing disagrees with another, through destroying or hindering that which agrees with it. Consequently love must needs precede hatred; and nothing is hated, save through being contrary to a suitable thing which is loved.” source