The very fact that a man hates whatever is opposed to the object of his love, is the effect of love.
 Thomas Aquinas, Summa Theologica (1274). copy citation

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Author Thomas Aquinas
Source Summa Theologica
Topic love hate
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

“But it does not arise, properly speaking, in the case of those things which, in their entirety, can be possessed by many: for no one envies another the knowledge of truth, which can be known entirely by many; except perhaps one may envy another his superiority in the knowledge of it. Reply Obj. 3: The very fact that a man hates whatever is opposed to the object of his love, is the effect of love. Hence zeal is set down as an effect of love rather than of hatred. ________________________ FIFTH ARTICLE [I-II, Q. 28, Art. 5] Whether Love Is a Passion That Wounds the Lover? Objection 1: It would seem that love wounds the lover.” source