Objection 1. It seems that love does not exist in God. For in God there are no passions. Now love is a passion. Therefore love is not in God.
Objection 2. Further, love, anger, sorrow and the like, are mutually divided against one another. But sorrow and anger are not attributed to God, unless by metaphor. Therefore neither is love attributed to Him.
Objection 3. Further, Dionysius says (Div. Nom. iv): "Love is a uniting and binding force." But this cannot take place in God, since He is simple. Therefore love does not exist in God.
On the contrary, It is written: "God is love" (John 4:16).
I answer that, We must needs assert that in God there is love: because love is the first movement of the will and of every appetitive faculty. For since the acts of the will and of every appetitive faculty tend towards good and evil, as to their proper objects: and since good is essentially and especially the object of the will and the appetite, whereas evil is only the object secondarily and indirectly, as opposed to good; it follows that the acts of the will and appetite that regard good must naturally be prior to those that regard evil; thus, for instance, joy is prior to sorrow, love to hate: because what exists of itself is always prior to that which exists through another. Again, the more universal is naturally prior to what is less so. Hence the intellect is first directed to universal truth; and in the second place to particular and special truths. Now there are certain acts of the will and appetite that regard good under some special condition, as joy and delight regard good present and possessed; whereas desire and hope regard good not as yet possessed. Love, however, regards good universally, whether possessed or not. Hence love is naturally the first act of the will and appetite; for which reason all the other appetite movements presuppose love, as their root and origin. For nobody desires anything nor rejoices in anything, except as a good that is loved: nor is anything an object of hate except as opposed to the object of love. Similarly, it is clear that sorrow, and other things like to it, must be referred to love as to their first principle. Hence, in whomsoever there is will and appetite, there must also be love: since if the first is wanting, all that follows is also wanting. Now it has been shown that will is in God (19, 1), and hence we must attribute love to Him.
Reply to Objection 1. The cognitive faculty does not move except through the medium of the appetitive: and just as in ourselves the universal reason moves through the medium of the particular reason, as stated in De Anima iii, 58,75, so in ourselves the intellectual appetite, or the will as it is called, moves through the medium of the sensitive appetite. Hence, in us the sensitive appetite is the proximate motive-force of our bodies. Some bodily change therefore always accompanies an act of the sensitive appetite, and this change affects especially the heart, which, as the Philosopher says (De part. animal. iii, 4), is the first principle of movement in animals. Therefore acts of the sensitive appetite, inasmuch as they have annexed to them some bodily change, are called passions; whereas acts of the will are not so called. Love, therefore, and joy and delight are passions; in so far as they denote acts of the intellective appetite, they are not passions. It is in this latter sense that they are in God. Hence the Philosopher says (Ethic. vii): "God rejoices by an operation that is one and simple," and for the same reason He loves without passion.
Reply to Objection 2. In the passions of the sensitive appetite there may be distinguished a certain material element--namely, the bodily change--and a certain formal element, which is on the part of the appetite. Thus in anger, as the Philosopher says (De Anima iii, 15,63,64), the material element is the kindling of the blood about the heart; but the formal, the appetite for revenge. Again, as regards the formal element of certain passions a certain imperfection is implied, as in desire, which is of the good we have not, and in sorrow, which is about the evil we have. This applies also to anger, which supposes sorrow. Certain other passions, however, as love and joy, imply no imperfection. Since therefore none of these can be attributed to God on their material side, as has been said (ad 1); neither can those that even on their formal side imply imperfection be attributed to Him; except metaphorically, and from likeness of effects, as already show (3, 2, ad 2; 19, 11). Whereas, those that do not imply imperfection, such as love and joy, can be properly predicated of God, though without attributing passion to Him, as said before (19, 11).
Reply to Objection 3. An act of love always tends towards two things; to the good that one wills, and to the person for whom one wills it: since to love a person is to wish that person good. Hence, inasmuch as we love ourselves, we wish ourselves good; and, so far as possible, union with that good. So love is called the unitive force, even in God, yet without implying composition; for the good that He wills for Himself, is no other than Himself, Who is good by His essence, as above shown (6, 1, 3). And by the fact that anyone loves another, he wills good to that other. Thus he puts the other, as it were, in the place of himself; and regards the good done to him as done to himself. So far love is a binding force, since it aggregates another to ourselves, and refers his good to our own. And then again the divine love is a binding force, inasmuch as God wills good to others; yet it implies no composition in God.
Source: Summa Theologica (New Advent)