Objection 1. It would seem that some other passion can be the cause of love. For the Philosopher (Ethic. viii, 3) says that some are loved for the sake of the pleasure they give. But pleasure is a passion. Therefore another passion is a cause of love.
Objection 2. Further, desire is a passion. But we love some because we desire to receive something from them: as happens in every friendship based on usefulness. Therefore another passion is a cause of love.
Objection 3. Further, Augustine says (De Trin. x, 1): "When we have no hope of getting a thing, we love it but half-heartedly or not at all, even if we see how beautiful it is." Therefore hope too is a cause of love.
On the contrary, All the other emotions of the soul are caused by love, as Augustine says (De Civ. Dei xiv, 7,9).
I answer that, There is no other passion of the soul that does not presuppose love of some kind. The reason is that every other passion of the soul implies either movement towards something, or rest in something. Now every movement towards something, or rest in something, arises from some kinship or aptness to that thing; and in this does love consist. Therefore it is not possible for any other passion of the soul to be universally the cause of every love. But it may happen that some other passion is the cause of some particular love: just as one good is the cause of another.
Reply to Objection 1. When a man loves a thing for the pleasure it affords, his love is indeed caused by pleasure; but that very pleasure is caused, in its turn, by another preceding love; for none takes pleasure save in that which is loved in some way.
Reply to Objection 2. Desire for a thing always presupposes love for that thing. But desire of one thing can be the cause of another thing's being loved; thus he that desires money, for this reason loves him from whom he receives it.
Reply to Objection 3. Hope causes or increases love; both by reason of pleasure, because it causes pleasure; and by reason of desire, because hope strengthens desire, since we do not desire so intensely that which we have no hope of receiving. Nevertheless hope itself is of a good that is loved.
Source: Summa Theologica (New Advent)