Objection 1. It would seem that irrational creatures also ought to be loved out of charity. For it is chiefly by charity that we are conformed to God. Now God loves irrational creatures out of charity, for He loves "all things that are" (Wisdom 11:25), and whatever He loves, He loves by Himself Who is charity. Therefore we also should love irrational creatures out of charity.
Objection 2. Further, charity is referred to God principally, and extends to other things as referable to God. Now just as the rational creature is referable to God, in as much as it bears the resemblance of image, so too, are the irrational creatures, in as much as they bear the resemblance of a trace [Cf. I, 45, 7]. Therefore charity extends also to irrational creatures.
Objection 3. Further, just as the object of charity is God. so is the object of faith. Now faith extends to irrational creatures, since we believe that heaven and earth were created by God, that the fishes and birds were brought forth out of the waters, and animals that walk, and plants, out of the earth. Therefore charity extends also to irrational creatures.
On the contrary, The love of charity extends to none but God and our neighbor. But the word neighbor cannot be extended to irrational creatures, since they have no fellowship with man in the rational life. Therefore charity does not extend to irrational creatures.
I answer that, According to what has been stated above (Question 13, Article 1) charity is a kind of friendship. Now the love of friendship is twofold: first, there is the love for the friend to whom our friendship is given, secondly, the love for those good things which we desire for our friend. With regard to the first, no irrational creature can be loved out of charity; and for three reasons. Two of these reasons refer in a general way to friendship, which cannot have an irrational creature for its object: first because friendship is towards one to whom we wish good things, while, properly speaking, we cannot wish good things to an irrational creature, because it is not competent, properly speaking, to possess good, this being proper to the rational creature which, through its free-will, is the master of its disposal of the good it possesses. Hence the Philosopher says (Phys. ii, 6) that we do not speak of good or evil befalling such like things, except metaphorically. Secondly, because all friendship is based on some fellowship in life; since "nothing is so proper to friendship as to live together," as the Philosopher proves (Ethic. viii, 5). Now irrational creatures can have no fellowship in human life which is regulated by reason. Hence friendship with irrational creatures is impossible, except metaphorically speaking. The third reason is proper to charity, for charity is based on the fellowship of everlasting happiness, to which the irrational creature cannot attain. Therefore we cannot have the friendship of charity towards an irrational creature.
Nevertheless we can love irrational creatures out of charity, if we regard them as the good things that we desire for others, in so far, to wit, as we wish for their preservation, to God's honor and man's use; thus too does God love them out of charity.
Wherefore the Reply to the First Objection is evident.
Reply to Objection 2. The likeness by way of trace does not confer the capacity for everlasting life, whereas the likeness of image does: and so the comparison fails.
Reply to Objection 3. Faith can extend to all that is in any way true, whereas the friendship of charity extends only to such things as have a natural capacity for everlasting life; wherefore the comparison fails.
Source: Summa Theologica (New Advent)