Objection 1. It would seem that we are not bound to love the angels out of charity. For, as Augustine says (De Doctr. Christ. i), charity is a twofold love: the love of God and of our neighbor. Now love of the angels is not contained in the love of God, since they are created substances; nor is it, seemingly, contained in the love of our neighbor, since they do not belong with us to a common species. Therefore we are not bound to love them out of charity.
Objection 2. Further, dumb animals have more in common with us than the angels have, since they belong to the same proximate genus as we do. But we have not charity towards dumb animals, as stated above (Article 3). Neither, therefore, have we towards the angels.
Objection 3. Further, nothing is so proper to friends as companionship with one another (Ethic. viii, 5). But the angels are not our companions; we cannot even see them. Therefore we are unable to give them the friendship of charity.
On the contrary, Augustine says (De Doctr. Christ. i, 30): "If the name of neighbor is given either to those whom we pity, or to those who pity us, it is evident that the precept binding us to love our neighbor includes also the holy angels from whom we receive many merciful favors."
I answer that, As stated above (Question 23, Article 1), the friendship of charity is founded upon the fellowship of everlasting happiness, in which men share in common with the angels. For it is written (Matthew 22:30) that "in the resurrection... men shall be as the angels of God in heaven." It is therefore evident that the friendship of charity extends also to the angels.
Reply to Objection 1. Our neighbor is not only one who is united to us in a common species, but also one who is united to us by sharing in the blessings pertaining to everlasting life, and it is on the latter fellowship that the friendship of charity is founded.
Reply to Objection 2. Dumb animals are united to us in the proximate genus, by reason of their sensitive nature; whereas we are partakers of everlasting happiness, by reason not of our sensitive nature but of our rational mind wherein we associate with the angels.
Reply to Objection 3. The companionship of the angels does not consist in outward fellowship, which we have in respect of our sensitive nature; it consists in a fellowship of the mind, imperfect indeed in this life, but perfect in heaven, as stated above (23, 1, ad 1).
Source: Summa Theologica (New Advent)