26.2 - i would call love the first perfection of the subject.
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.