Objection 1. It would seem that union is not an effect of love. For absence is incompatible with union. But love is compatible with absence; for the Apostle says (Galatians 4:18): "Be zealous for that which is good in a good thing always" (speaking of himself, according to a gloss), "and not only when I am present with you." Therefore union is not an effect of love.
Objection 2. Further, every union is either according to essence, thus form is united to matter, accident to subject, and a part to the whole, or to another part in order to make up the whole: or according to likeness, in genus, species, or accident. But love does not cause union of essence; else love could not be between things essentially distinct. On the other hand, love does not cause union of likeness, but rather is caused by it, as stated above (Question 27, Article 3). Therefore union is not an effect of love.
Objection 3. Further, the sense in act is the sensible in act, and the intellect in act is the thing actually understood. But the lover in act is not the beloved in act. Therefore union is the effect of knowledge rather than of love.
On the contrary, Dionysius says (Div. Nom. iv) that every love is a "unitive love."
I answer that, The union of lover and beloved is twofold. The first is real union; for instance, when the beloved is present with the lover. The second is union of affection: and this union must be considered in relation to the preceding apprehension; since movement of the appetite follows apprehension. Now love being twofold, viz. love of concupiscence and love of friendship; each of these arises from a kind of apprehension of the oneness of the thing loved with the lover. For when we love a thing, by desiring it, we apprehend it as belonging to our well-being. In like manner when a man loves another with the love of friendship, he wills good to him, just as he wills good to himself: wherefore he apprehends him as his other self, in so far, to wit, as he wills good to him as to himself. Hence a friend is called a man's "other self" (Ethic. ix, 4), and Augustine says (Confess. iv, 6), "Well did one say to his friend: Thou half of my soul."
The first of these unions is caused "effectively" by love; because love moves man to desire and seek the presence of the beloved, as of something suitable and belonging to him. The second union is caused "formally" by love; because love itself is this union or bond. In this sense Augustine says (De Trin. viii, 10) that "love is a vital principle uniting, or seeking to unite two together, the lover, to wit, and the beloved." For in describing it as "uniting" he refers to the union of affection, without which there is no love: and in saying that "it seeks to unite," he refers to real union.
Reply to Objection 1. This argument is true of real union. That is necessary to pleasure as being its cause; desire implies the real absence of the beloved: but love remains whether the beloved be absent or present.
Reply to Objection 2. Union has a threefold relation to love. There is union which causes love; and this is substantial union, as regards the love with which one loves oneself; while as regards the love wherewith one loves other things, it is the union of likeness, as stated above (Question 27, Article 3). There is also a union which is essentially love itself. This union is according to a bond of affection, and is likened to substantial union, inasmuch as the lover stands to the object of his love, as to himself, if it be love of friendship; as to something belonging to himself, if it be love of concupiscence. Again there is a union, which is the effect of love. This is real union, which the lover seeks with the object of his love. Moreover this union is in keeping with the demands of love: for as the Philosopher relates (Polit. ii, 1), "Aristophanes stated that lovers would wish to be united both into one," but since "this would result in either one or both being destroyed," they seek a suitable and becoming union--to live together, speak together, and be united together in other like things.
Reply to Objection 3. Knowledge is perfected by the thing known being united, through its likeness, to the knower. But the effect of love is that the thing itself which is loved, is, in a way, united to the lover, as stated above. Consequently the union caused by love is closer than that which is caused by knowledge.
Source: Summa Theologica (New Advent)