Objection 1. It would seem that likeness is not a cause of love. For the same thing is not the cause of contraries. But likeness is the cause of hatred; for it is written (Proverbs 13:10) that "among the proud there are always contentions"; and the Philosopher says (Ethic. viii, 1) that "potters quarrel with one another." Therefore likeness is not a cause of love.
Objection 2. Further, Augustine says (Confess. iv, 14) that "a man loves in another that which he would not be himself: thus he loves an actor, but would not himself be an actor." But it would not be so, if likeness were the proper cause of love; for in that case a man would love in another, that which he possesses himself, or would like to possess. Therefore likeness is not a cause of love.
Objection 3. Further, everyone loves that which he needs, even if he have it not: thus a sick man loves health, and a poor man loves riches. But in so far as he needs them and lacks them, he is unlike them. Therefore not only likeness but also unlikeness is a cause of love.
Objection 4. Further, the Philosopher says (Rhet. ii, 4) that "we love those who bestow money and health on us; and also those who retain their friendship for the dead." But all are not such. Therefore likeness is not a cause of love.
On the contrary, It is written (Sirach 13:19): "Every beast loveth its like."
I answer that, Likeness, properly speaking, is a cause of love. But it must be observed that likeness between things is twofold. One kind of likeness arises from each thing having the same quality actually: for example, two things possessing the quality of whiteness are said to be alike. Another kind of likeness arises from one thing having potentially and by way of inclination, a quality which the other has actually: thus we may say that a heavy body existing outside its proper place is like another heavy body that exists in its proper place: or again, according as potentiality bears a resemblance to its act; since act is contained, in a manner, in the potentiality itself.
Accordingly the first kind of likeness causes love of friendship or well-being. For the very fact that two men are alike, having, as it were, one form, makes them to be, in a manner, one in that form: thus two men are one thing in the species of humanity, and two white men are one thing in whiteness. Hence the affections of one tend to the other, as being one with him; and he wishes good to him as to himself. But the second kind of likeness causes love of concupiscence, or friendship founded on usefulness or pleasure: because whatever is in potentiality, as such, has the desire for its act; and it takes pleasure in its realization, if it be a sentient and cognitive being.
Now it has been stated above (Question 26, Article 4), that in the love of concupiscence, the lover, properly speaking, loves himself, in willing the good that he desires. But a man loves himself more than another: because he is one with himself substantially, whereas with another he is one only in the likeness of some form. Consequently, if this other's likeness to him arising from the participation of a form, hinders him from gaining the good that he loves, he becomes hateful to him, not for being like him, but for hindering him from gaining his own good. This is why "potters quarrel among themselves," because they hinder one another's gain: and why "there are contentions among the proud," because they hinder one another in attaining the position they covet.
Hence the Reply to the First Objection is evident.
Reply to Objection 2. Even when a man loves in another what he loves not in himself, there is a certain likeness of proportion: because as the latter is to that which is loved in him, so is the former to that which he loves in himself: for instance, if a good singer love a good writer, we can see a likeness of proportion, inasmuch as each one has that which is becoming to him in respect of his art.
Reply to Objection 3. He that loves what he needs, bears a likeness to what he loves, as potentiality bears a likeness to its act, as stated above.
Reply to Objection 4. According to the same likeness of potentiality to its act, the illiberal man loves the man who is liberal, in so far as he expects from him something which he desires. The same applies to the man who is constant in his friendship as compared to one who is inconstant. For in either case friendship seems to be based on usefulness. We might also say that although not all men have these virtues in the complete habit, yet they have them according to certain seminal principles in the reason, in force of which principles the man who is not virtuous loves the virtuous man, as being in conformity with his own natural reason.
Source: Summa Theologica (New Advent)