Objection 1. It would seem that love is not a passion. For no power is a passion. But every love is a power, as Dionysius says (Div. Nom. iv). Therefore love is not a passion.
Objection 2. Further, love is a kind of union or bond, as Augustine says (De Trin. viii, 10). But a union or bond is not a passion, but rather a relation. Therefore love is not a passion.
Objection 3. Further, Damascene says (De Fide Orth. ii, 22) that passion is a movement. But love does not imply the movement of the appetite; for this is desire, of which movement love is the principle. Therefore love is not a passion.
On the contrary, The Philosopher says (Ethic. viii, 5) that "love is a passion."
I answer that, Passion is the effect of the agent on the patient. Now a natural agent produces a twofold effect on the patient: for in the first place it gives it the form; and secondly it gives it the movement that results from the form. Thus the generator gives the generated body both weight and the movement resulting from weight: so that weight, from being the principle of movement to the place, which is connatural to that body by reason of its weight, can, in a way, be called "natural love." In the same way the appetible object gives the appetite, first, a certain adaptation to itself, which consists in complacency in that object; and from this follows movement towards the appetible object. For "the appetitive movement is circular," as stated in De Anima iii, 10; because the appetible object moves the appetite, introducing itself, as it were, into its intention; while the appetite moves towards the realization of the appetible object, so that the movement ends where it began. Accordingly, the first change wrought in the appetite by the appetible object is called "love," and is nothing else than complacency in that object; and from this complacency results a movement towards that same object, and this movement is "desire"; and lastly, there is rest which is "joy." Since, therefore, love consists in a change wrought in the appetite by the appetible object, it is evident that love is a passion: properly so called, according as it is in the concupiscible faculty; in a wider and extended sense, according as it is in the will.
Reply to Objection 1. Since power denotes a principle of movement or action, Dionysius calls love a power, in so far as it is a principle of movement in the appetite.
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.
Source: Summa Theologica (New Advent)