Amor est causa timoris quia omnis timor causatur ex hoc quod aliquid amamus: amor est causa eius per modum materialis dispositionis.
Love is the cause of fear because all fear is caused by our loving something: i.e., love causes fear by way of material disposition.
Obiecta passionum animae se habent ad eas tanquam formae ad res naturales vel artificiales, quia passiones animae speciem recipiunt ab obiectis, sicut res praedictae a suis formis. Sicut igitur quidquid est causa formae, est causa rei constitutae per ipsam; ita etiam quidquid, et quocumque modo, est causa obiecti, est causa passionis.
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.
Contingit autem aliquid esse causam obiecti vel per modum causae efficientis, vel per modum dispositionis materialis. Sicut obiectum delectationis est bonum apparens conveniens coniunctum, cuius causa efficiens est illud quod facit coniunctionem, vel quod facit convenientiam vel bonitatem, vel apparentiam huiusmodi boni; causa autem per modum dispositionis materialis, est habitus, vel quaecumque dispositio secundum quam fit alicui conveniens aut apparens illud bonum quod est ei coniunctum.
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 according to which this conjoined good becomes suitable or is apprehended as such.
Sic igitur, in proposito, obiectum timoris est aestimatum malum futurum propinquum cui resisti de facili non potest. Et ideo illud quod potest inferre tale malum, est causa effectiva obiecti timoris, et per consequens ipsius timoris. Illud autem per quod aliquis ita disponitur ut aliquid sit ei tale, est causa timoris, et obiecti eius, per modum dispositionis materialis.
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.
Et hoc modo amor est causa timoris: ex hoc enim quod aliquis amat aliquod bonum, sequitur quod privativum talis boni sit ei malum, et per consequens quod timeat ipsum tanquam malum.
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.
Augustinus dicit, in libro octoginta trium quaest., "nulli dubium est non aliam esse metuendi causam, nisi ne id quod amamus, aut adeptum amittamus, aut non adipiscamur speratum".
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."