Objection 1. It would seem that love is not a cause of hatred. For "the opposite members of a division are naturally simultaneous" (Praedic. x). But love and hatred are opposite members of a division, since they are contrary to one another. Therefore they are naturally simultaneous. Therefore love is not the cause of hatred.
Objection 2. Further, of two contraries, one is not the cause of the other. But love and hatred are contraries. Therefore love is not the cause of hatred.
Objection 3. Further, that which follows is not the cause of that which precedes. But hatred precedes love, seemingly: since hatred implies a turning away from evil, whereas love implies a turning towards good. 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 (Article 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. And hence it is that every hatred is caused by love.
Reply to Objection 1. The opposite members of a division are sometimes naturally simultaneous, both really and logically; e.g. two species of animal, or two species of color. Sometimes they are simultaneous logically, while, in reality, one precedes, and causes the other; e.g. the species of numbers, figures and movements. Sometimes they are not simultaneous either really or logically; e.g. substance and accident; for substance is in reality the cause of accident; and being is predicated of substance before it is predicated of accident, by a priority of reason, because it is not predicated of accident except inasmuch as the latter is in substance. Now love and hatred are naturally simultaneous, logically but not really. Wherefore nothing hinders love from being the cause of hatred.
Reply to Objection 2. Love and hatred are contraries if considered in respect of the same thing. But if taken in respect of contraries, they are not themselves contrary, but consequent to one another: for it amounts to the same that one love a certain thing, or that one hate its contrary. Thus love of one thing is the cause of one's hating its contrary.
Reply to Objection 3. In the order of execution, the turning away from one term precedes the turning towards the other. But the reverse is the case in the order of intention: since approach to one term is the reason for turning away from the other. Now the appetitive movement belongs rather to the order of intention than to that of execution. Wherefore love precedes hatred: because each is an appetitive movement.
Source: Summa Theologica (New Advent)