Objection 1. It would seem that good is not the only cause of love. For good does not cause love, except because it is loved. But it happens that evil also is loved, according to Psalm 10:6: "He that loveth iniquity, hateth his own soul": else, every love would be good. Therefore good is not the only cause of love.
Objection 2. Further, the Philosopher says (Rhet. ii, 4) that "we love those who acknowledge their evils." Therefore it seems that evil is the cause of love.
Objection 3. Further, Dionysius says (Div. Nom. iv) that not "the good" only but also "the beautiful is beloved by all."
On the contrary, Augustine says (De Trin. viii, 3): "Assuredly the good alone is beloved." Therefore good alone is the cause of love.
I answer that, As stated above (Question 26, Article 1), Love belongs to the appetitive power which is a passive faculty. Wherefore its object stands in relation to it as the cause of its movement or act. Therefore the cause of love must needs be love's object. Now the proper object of love is the good; because, as stated above (26, 1,2), love implies a certain connaturalness or complacency of the lover for the thing beloved, and to everything, that thing is a good, which is akin and proportionate to it. It follows, therefore, that good is the proper cause of love.
Reply to Objection 1. Evil is never loved except under the aspect of good, that is to say, in so far as it is good in some respect, and is considered as being good simply. And thus a certain love is evil, in so far as it tends to that which is not simply a true good. It is in this way that man "loves iniquity," inasmuch as, by means of iniquity, some good is gained; pleasure, for instance, or money, or such like.
Reply to Objection 2. Those who acknowledge their evils, are beloved, not for their evils, but because they acknowledge them, for it is a good thing to acknowledge one's faults, in so far as it excludes insincerity or hypocrisy.
Reply to Objection 3. The beautiful is the same as the good, and they differ in aspect only. For since good is what all seek, the notion of good is that which calms the desire; while the notion of the beautiful is that which calms the desire, by being seen or known. Consequently those senses chiefly regard the beautiful, which are the most cognitive, viz. sight and hearing, as ministering to reason; for we speak of beautiful sights and beautiful sounds. But in reference to the other objects of the other senses, we do not use the expression "beautiful," for we do not speak of beautiful tastes, and beautiful odors. Thus it is evident that beauty adds to goodness a relation to the cognitive faculty: so that "good" means that which simply pleases the appetite; while the "beautiful" is something pleasant to apprehend.
Source: Summa Theologica (New Advent)