(From q. 26.2)
Reply to Objection 2. Union belongs to love in so far as by reason of the complacency of the appetite, the lover stands in relation to that which he loves, as though it were himself or part of himself. Hence it is clear that love is not the very relation of union, but that union is a result of love. Hence, too, Dionysius says that "love is a unitive force" (Div. Nom. iv), and the Philosopher says (Polit. ii, 1) that union is the work of love.
Reply to Objection 3. Although love does not denote the movement of the appetite in tending towards the appetible object, yet it denotes that movement whereby the appetite is changed by the appetible object, so as to have complacency therein.