In the first part of the second part of his Summa Theologicae, Thomas Aquinas talks about the irascible passions, in particular, fear. Aquinas defines fear, discusses the object of it, cause of it and the effects of it. Aquinas brings up two points of discussion to determine the cause of fear: the first, love, the second, defect.
In his analysis that love is indeed a cause of fear, Aquinas refers to Augustine, who said "(Questions. 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." Aquinas explains this in fuller detail. Aquinas states that whatever is the cause of the object is the cause of passion, and that which can inflict evil equals the cause of fear. When someone has love for an object, and that object is taken, he sees it as an evil doing. The infliction of evil equals fear, so essentially, love leads to fear. .
The objections Aquinas contemplates are that: one, fear is not caused by love, but reversibly, fear is the cause of love. Two, fear is not caused by love, instead it is caused by our hatred of something due to threat of evil. Three, our own actions are not fearful, and since what we love we do with our heart, love cannot cause fear. Aquinas responds to this point by point. For the first objection, Aquinas says that fear regards evil as being contrary to a loved good, therefore, fear is born of love. However, in some circumstances, fear can give rise to love. The example Aquinas gives is "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])." In response to the second objection, Aquinas disagrees with the idea that an individual's fear is due to hate directed towards an evil. Aquinas states that although one may start off hating an object that is cause of the evil, one can begin to love that object when he begins to hope for good from it.