I answer that, God is the reason for our loving our neighbor out of charity, as stated above (Question [25], Article [1]). When therefore it is asked which is better or more meritorious, to love one's friend or one's enemy, these two loves may be compared in two ways, first, on the part of our neighbor whom we love, secondly, on the part of the reason for which we love him.
In the first way, love of one's friend surpasses love of one's enemy, because a friend is both better and more closely united to us, so that he is a more suitable matter of love and consequently the act of love that passes over this matter, is better, and therefore its opposite is worse, for it is worse to hate a friend than an enemy.
In the second way, however, it is better to love one's enemy than one's friend, and this for two reasons. First, because it is possible to love one's friend for another reason than God, whereas God is the only reason for loving one's enemy. Secondly, because if we suppose that both are loved for God, our love for God is proved to be all the stronger through carrying a man's affections to things which are furthest from him, namely, to the love of his enemies, even as the power of a furnace is proved to be the stronger, according as it throws its heat to more distant objects. Hence our love for God is proved to be so much the stronger, as the more difficult are the things we accomplish for its sake, just as the power of fire is so much the stronger, as it is able to set fire to a less inflammable matter.
Yet just as the same fire acts with greater force on what is near than on what is distant, so too, charity loves with greater fervor those who are united to us than those who are far removed; and in this respect the love of friends, considered in itself, is more ardent and better than the love of one's enemy.