Objection 1. It would seem that love is not the cause of fear. For that which leads to a thing is its cause. But "fear leads to the love of charity" as Augustine says on the canonical epistle of John (Tract. ix). Therefore fear is the cause of love, and not conversely.
Objection 2. Further, the Philosopher says (Rhet. ii, 5) that "those are feared most from whom we dread the advent of some evil." But the dread of evil being caused by someone, makes us hate rather than love him. Therefore fear is caused by hate rather than by love.
Objection 3. Further, it has been stated above (Question 42, Article 3) that those things which occur by our own doing are not fearful. But that which we do from love, is done from our inmost heart. Therefore fear is not caused by love.
On the contrary, Augustine says (QQ. 83, qu. 33): "There can be no doubt that there is no cause for fear save the loss of what we love, when we possess it, or the failure to obtain what we hope for." Therefore all fear is caused by our loving something: and consequently love is the cause of fear.
I answer that, The objects of the soul's passions stand in relation thereto as the forms to things natural or artificial: because the passions of the soul take their species from their objects, as the aforesaid things do from their forms. Therefore, just as whatever is a cause of the form, is a cause of the thing constituted by that form, so whatever is a cause, in any way whatever, of the object, is a cause of the passion. Now a thing may be a cause of the object, either by way of efficient cause, or by way of material disposition. Thus the object of pleasure is good apprehended as suitable and conjoined: and its efficient cause is that which causes the conjunction, or the suitableness, or goodness, or apprehension of that good thing; while its cause by way of material disposition, is a habit or any sort of disposition by reason of which this conjoined good becomes suitable or is apprehended as such.
Accordingly, as to the matter in question, the object of fear is something reckoned as an evil to come, near at hand and difficult to avoid. Therefore that which can inflict such an evil, is the efficient cause of the object of fear, and, consequently, of fear itself. While that which renders a man so disposed that thing is such an evil to him, is a cause of fear and of its object, by way of material disposition. And thus it is that love causes fear: since it is through his loving a certain good, that whatever deprives a man of that good is an evil to him, and that consequently he fears it as an evil.
Reply to Objection 1. As stated above (Question 42, Article 1), fear, of itself and in the first place, regards the evil from which it recoils as being contrary to some loved good: and thus fear, of itself, is born of love. But, in the second place, it regards the cause from which that evil ensues: so that sometimes, accidentally, fear gives rise to love; in so far as, for instance, through fear of God's punishments, man keeps His commandments, and thus begins to hope, while hope leads to love, as stated above (Question 40, Article 7).
Reply to Objection 2. He, from whom evil is expected, is indeed hated at first; but afterwards, when once we begin to hope for good from him, we begin to love him. But the good, the contrary evil of which is feared, was loved from the beginning.
Reply to Objection 3. This argument is true of that which is the efficient cause of the evil to be feared: whereas love causes fear by way of material disposition, as stated above.
Source: Summa Theologica (New Advent)