Objection 1. It would seem that hatred is stronger than love. For Augustine says (QQ. 83, qu. 36): "There is no one who does not flee from pain, more than he desires pleasure." But flight from pain pertains to hatred; while desire for pleasure belongs to love. Therefore hatred is stronger than love.
Objection 2. Further, the weaker is overcome by the stronger. But love is overcome by hatred: when, that is to say, love is turned into hatred. Therefore hatred is stronger than love.
Objection 3. Further, the emotions of the soul are shown by their effects. But man insists more on repelling what is hateful, than on seeking what is pleasant: thus also irrational animals refrain from pleasure for fear of the whip, as Augustine instances (QQ. 83, qu. 36). Therefore hatred is stronger than love.
On the contrary, Good is stronger than evil; because "evil does nothing except in virtue of good," as Dionysius says (Div. Nom. iv). But hatred and love differ according to the difference of good and evil. Therefore love is stronger than hatred.
I answer that, It is impossible for an effect to be stronger than its cause. Now every hatred arises from some love as its cause, as above stated (2). Therefore it is impossible for hatred to be stronger than love absolutely.
But furthermore, love must needs be, absolutely speaking, stronger than hatred. Because a thing is moved to the end more strongly than to the means. Now turning away from evil is directed as a means to the gaining of good. Wherefore, absolutely speaking, the soul's movement in respect of good is stronger than its movement in respect of evil.
Nevertheless hatred sometimes seems to be stronger than love, for two reasons. First, because hatred is more keenly felt than love. For, since the sensitive perception is accompanied by a certain impression; when once the impression has been received it is not felt so keenly as in the moment of receiving it. Hence the heat of a hectic fever, though greater, is nevertheless not felt so much as the heat of tertian fever; because the heat of the hectic fever is habitual and like a second nature. For this reason, love is felt more keenly in the absence of the object loved; thus Augustine says (De Trin. x, 12) that "love is felt more keenly when we lack what we love." And for the same reason, the unbecomingness of that which is hated is felt more keenly than the becomingness of that which is loved. Secondly, because comparison is made between a hatred and a love which are not mutually corresponding. Because, according to different degrees of good there are different degrees of love to which correspond different degrees of hatred. Wherefore a hatred that corresponds to a greater love, moves us more than a lesser love.
Hence it is clear how to reply to the First Objection. For the love of pleasure is less than the love of self-preservation, to which corresponds flight from pain. Wherefore we flee from pain more than we love pleasure.
Source: Summa Theologica (New Advent)