Thomas Aquinas

“ And still greater would be the defect, if perception of the evil, or love of the good whose contrary is feared, were entirely absent.
But as to the second, power and strength are, of themselves, the cause of fear: because it is owing to the fact that the cause apprehended as harmful is powerful, that its effect cannot be repulsed. It may happen, however, in this respect, that some defect causes fear accidentally, in so far as owing to some defect someone wishes to hurt another ”

“ 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. ”

“ 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. ”