Objection 1. It would seem that zeal is not an effect of love. For zeal is a beginning of contention; wherefore it is written (1 Corinthians 3:3): "Whereas there is among you zeal [Douay: 'envying'] and contention," etc. But contention is incompatible with love. Therefore zeal is not an effect of love.
Objection 2. Further, the object of love is the good, which communicates itself to others. But zeal is opposed to communication; since it seems an effect of zeal, that a man refuses to share the object of his love with another: thus husbands are said to be jealous of [zelare] their wives, because they will not share them with others. Therefore zeal is not an effect of love.
Objection 3. Further, there is no zeal without hatred, as neither is there without love: for it is written (Psalm 72:3): "I had a zeal on occasion of the wicked." Therefore it should not be set down as an effect of love any more than of hatred.
On the contrary, Dionysius says (Div. Nom. iv): "God is said to be a zealot, on account of his great love for all things."
I answer that, Zeal, whatever way we take it, arises from the intensity of love. For it is evident that the more intensely a power tends to anything, the more vigorously it withstands opposition or resistance. Since therefore love is "a movement towards the object loved," as Augustine says (QQ. 83, qu. 35), an intense love seeks to remove everything that opposes it.
But this happens in different ways according to love of concupiscence, and love of friendship. For in love of concupiscence he who desires something intensely, is moved against all that hinders his gaining or quietly enjoying the object of his love. It is thus that husbands are said to be jealous of their wives, lest association with others prove a hindrance to their exclusive individual rights. In like manner those who seek to excel, are moved against those who seem to excel, as though these were a hindrance to their excelling. And this is the zeal of envy, of which it is written (Psalm 36:1): "Be not emulous of evil doers, nor envy [zelaveris] them that work iniquity."
On the other hand, love of friendship seeks the friend's good: wherefore, when it is intense, it causes a man to be moved against everything that opposes the friend's good. In this respect, a man is said to be zealous on behalf of his friend, when he makes a point of repelling whatever may be said or done against the friend's good. In this way, too, a man is said to be zealous on God's behalf, when he endeavors, to the best of his means, to repel whatever is contrary to the honor or will of God; according to 1 Kings 19:14: "With zeal I have been zealous for the Lord of hosts." Again on the words of John 2:17: "The zeal of Thy house hath eaten me up," a gloss says that "a man is eaten up with a good zeal, who strives to remedy whatever evil he perceives; and if he cannot, bears with it and laments it."
Reply to Objection 1. The Apostle is speaking in this passage of the zeal of envy; which is indeed the cause of contention, not against the object of love, but for it, and against that which is opposed to it.
Reply to Objection 2. Good is loved inasmuch as it can be communicated to the lover. Consequently whatever hinders the perfection of this communication, becomes hateful. Thus zeal arises from love of good. But through defect of goodness, it happens that certain small goods cannot, in their entirety, be possessed by many at the same time: and from the love of such things arises the zeal of envy. But it does not arise, properly speaking, in the case of those things which, in their entirety, can be possessed by many: for no one envies another the knowledge of truth, which can be known entirely by many; except perhaps one may envy another his superiority in the knowledge of it.
Reply to Objection 3. The very fact that a man hates whatever is opposed to the object of his love, is the effect of love. Hence zeal is set down as an effect of love rather than of hatred.
Source: Summa Theologica (New Advent)