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 (Q. 26, A. 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 Obj. 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 Obj. 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.