Objection 1. It would seem that love is not in the concupiscible power. For it is written (Wisdom 8:2): "Her," namely wisdom, "have I loved, and have sought her out from my youth." But the concupiscible power, being a part of the sensitive appetite, cannot tend to wisdom, which is not apprehended by the senses. Therefore love is not in the concupiscible power.
Objection 2. Further, love seems to be identified with every passion: for Augustine says (De Civ. Dei xiv, 7): "Love, yearning for the object beloved, is desire; having and enjoying it, is joy; fleeing what is contrary to it, is fear; and feeling what is contrary to it, is sadness." But not every passion is in the concupiscible power; indeed, fear, which is mentioned in this passage, is in the irascible power. Therefore we must not say absolutely that love is in the concupiscible power.
Objection 3. Further, Dionysius (Div. Nom. iv) mentions a "natural love." But natural love seems to pertain rather to the natural powers, which belong to the vegetal soul. Therefore love is not simply in the concupiscible power.
On the contrary, The Philosopher says (Topic. ii, 7) that "love is in the concupiscible power."
I answer that, Love is something pertaining to the appetite; since good is the object of both. Wherefore love differs according to the difference of appetites. For there is an appetite which arises from an apprehension existing, not in the subject of the appetite, but in some other: and this is called the "natural appetite." Because natural things seek what is suitable to them according to their nature, by reason of an apprehension which is not in them, but in the Author of their nature, as stated in the I, 6, 1, ad 2; I, 103, 1, ad 1,3. And there is another appetite arising from an apprehension in the subject of the appetite, but from necessity and not from free-will. Such is, in irrational animals, the "sensitive appetite," which, however, in man, has a certain share of liberty, in so far as it obeys reason. Again, there is another appetite following freely from an apprehension in the subject of the appetite. And this is the rational or intellectual appetite, which is called the "will."
Now in each of these appetites, the name "love" is given to the principle movement towards the end loved. In the natural appetite the principle of this movement is the appetitive subject's connaturalness with the thing to which it tends, and may be called "natural love": thus the connaturalness of a heavy body for the centre, is by reason of its weight and may be called "natural love." In like manner the aptitude of the sensitive appetite or of the will to some good, that is to say, its very complacency in good is called "sensitive love," or "intellectual" or "rational love." So that sensitive love is in the sensitive appetite, just as intellectual love is in the intellectual appetite. And it belongs to the concupiscible power, because it regards good absolutely, and not under the aspect of difficulty, which is the object of the irascible faculty.
Reply to Objection 1. The words quoted refer to intellectual or rational love.
Reply to Objection 2. Love is spoken of as being fear, joy, desire and sadness, not essentially but causally.
Reply to Objection 3. Natural love is not only in the powers of the vegetal soul, but in all the soul's powers, and also in all the parts of the body, and universally in all things: because, as Dionysius says (Div. Nom. iv), "Beauty and goodness are beloved by all things"; since each single thing has a connaturalness with that which is naturally suitable to it.
Source: Summa Theologica (New Advent)