Objection 1. It would seem that knowledge is not a cause of love. For it is due to love that a thing is sought. But some things are sought without being known, for instance, the sciences; for since "to have them is the same as to know them," as Augustine says (Q83, qu. 35), if we knew them we should have them, and should not seek them. Therefore knowledge is not the cause of love.
Objection 2. Further, to love what we know not seems like loving something more than we know it. But some things are loved more than they are known: thus in this life God can be loved in Himself, but cannot be known in Himself. Therefore knowledge is not the cause of love.
Objection 3. Further, if knowledge were the cause of love, there would be no love, where there is no knowledge. But in all things there is love, as Dionysius says (Div. Nom. iv); whereas there is not knowledge in all things. Therefore knowledge is not the cause of love.
On the contrary, Augustine proves (De Trin. x, 1,2) that "none can love what he does not know."
I answer that, As stated above (Article 1), good is the cause of love, as being its object. But good is not the object of the appetite, except as apprehended. And therefore love demands some apprehension of the good that is loved. For this reason the Philosopher (Ethic. ix, 5,12) says that bodily sight is the beginning of sensitive love: and in like manner the contemplation of spiritual beauty or goodness is the beginning of spiritual love. Accordingly knowledge is the cause of love for the same reason as good is, which can be loved only if known.
Reply to Objection 1. He who seeks science, is not entirely without knowledge thereof: but knows something about it already in some respect, either in a general way, or in some one of its effects, or from having heard it commended, as Augustine says (De Trin. x, 1,2). But to have it is not to know it thus, but to know it perfectly.
Reply to Objection 2. Something is required for the perfection of knowledge, that is not requisite for the perfection of love. For knowledge belongs to the reason, whose function it is to distinguish things which in reality are united, and to unite together, after a fashion, things that are distinct, by comparing one with another. Consequently the perfection of knowledge requires that man should know distinctly all that is in a thing, such as its parts, powers, and properties. On the other hand, love is in the appetitive power, which regards a thing as it is in itself: wherefore it suffices, for the perfection of love, that a thing be loved according as it is known in itself. Hence it is, therefore, that a thing is loved more than it is known; since it can be loved perfectly, even without being perfectly known. This is most evident in regard to the sciences, which some love through having a certain general knowledge of them: for instance, they know that rhetoric is a science that enables man to persuade others; and this is what they love in rhetoric. The same applies to the love of God.
Reply to Objection 3. Even natural love, which is in all things, is caused by a kind of knowledge, not indeed existing in natural things themselves, but in Him Who created their nature, as stated above (26, 1; cf. I, 6, 1, ad 2).
Source: Summa Theologica (New Advent)