Objection 1. It would seem more meritorious to love an enemy than to love a friend. For it is written (Matthew 5:46): "If you love them that love you, what reward shall you have?" Therefore it is not deserving of reward to love one's friend: whereas, as the same passage proves, to love one's enemy is deserving of a reward. Therefore it is more meritorious to love one's enemy than to love one's friend.
Objection 2. Further, an act is the more meritorious through proceeding from a greater charity. But it belongs to the perfect children of God to love their enemies, whereas those also who have imperfect charity love their friends. Therefore it is more meritorious to love one's enemy than to love one's friend.
Objection 3. Further, where there is more effort for good, there seems to be more merit, since "every man shall receive his own reward according to his own labor" (1 Corinthians 3:8). Now a man has to make a greater effort to love his enemy than to love his friend, because it is more difficult. Therefore it seems more meritorious to love one's enemy than to love one's friend.
On the contrary, The better an action is, the more meritorious it is. Now it is better to love one's friend, since it is better to love a better man, and the friend who loves you is better than the enemy who hates you. Therefore it is more meritorious to love one's friend than to love one's enemy.
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.
Reply to Objection 1. The words of Our Lord must be taken in their strict sense: because the love of one's friends is not meritorious in God's sight when we love them merely because they are our friends: and this would seem to be the case when we love our friends in such a way that we love not our enemies. On the other hand the love of our friends is meritorious, if we love them for God's sake, and not merely because they are our friends.
The Reply to the other Objections is evident from what has been said in the article, because the two arguments that follow consider the reason for loving, while the last considers the question on the part of those who are loved.
Source: Summa Theologica (New Advent)