Objection 1. It would seem that charity does not require us to love our enemies. For Augustine says (Enchiridion lxxiii) that "this great good," namely, the love of our enemies, is "not so universal in its application, as the object of our petition when we say: Forgive us our trespasses." Now no one is forgiven sin without he have charity, because, according to Proverbs 10:12, "charity covereth all sins." Therefore charity does not require that we should love our enemies.
Objection 2. Further, charity does not do away with nature. Now everything, even an irrational being, naturally hates its contrary, as a lamb hates a wolf, and water fire. Therefore charity does not make us love our enemies.
Objection 3. Further, charity "doth nothing perversely" (1 Corinthians 13:4). Now it seems perverse to love one's enemies, as it would be to hate one's friends: hence Joab upbraided David by saying (2 Samuel 19:6): "Thou lovest them that hate thee, and thou hatest them that love thee." Therefore charity does not make us love our enemies.
On the contrary, Our Lord said (Matthew 4:44): "Love your enemies."
I answer that, Love of one's enemies may be understood in three ways. First, as though we were to love our enemies as such: this is perverse, and contrary to charity, since it implies love of that which is evil in another.
Secondly love of one's enemies may mean that we love them as to their nature, but in general: and in this sense charity requires that we should love our enemies, namely, that in loving God and our neighbor, we should not exclude our enemies from the love given to our neighbor in general.
Thirdly, love of one's enemies may be considered as specially directed to them, namely, that we should have a special movement of love towards our enemies. Charity does not require this absolutely, because it does not require that we should have a special movement of love to every individual man, since this would be impossible. Nevertheless charity does require this, in respect of our being prepared in mind, namely, that we should be ready to love our enemies individually, if the necessity were to occur. That man should actually do so, and love his enemy for God's sake, without it being necessary for him to do so, belongs to the perfection of charity. For since man loves his neighbor, out of charity, for God's sake, the more he loves God, the more does he put enmities aside and show love towards his neighbor: thus if we loved a certain man very much, we would love his children though they were unfriendly towards us. This is the sense in which Augustine speaks in the passage quoted in the First Objection, the Reply to which is therefore evident.
Reply to Objection 2. Everything naturally hates its contrary as such. Now our enemies are contrary to us, as enemies, wherefore this itself should be hateful to us, for their enmity should displease us. They are not, however, contrary to us, as men and capable of happiness: and it is as such that we are bound to love them.
Reply to Objection 3. It is wrong to love one's enemies as such: charity does not do this, as stated above.
Source: Summa Theologica (New Advent)