The love of friendship seeks the good of the friend; therefore, when it is intense it leads a man to act against everything that is opposed to the good of the friend.
Gaudium Press English Edition
Newsdesk (13/06/2022 11:30 PM, Gaudium Press) “Jealousy, in whatever sense, comes from the intensity of love. Now, it is evident that the more intensely a power tends towards something, the more strongly it repels that which is contrary and incompatible with it”. Thus, since love is “a movement towards the beloved”, as Augustine says, intense love seeks to exclude everything that is contrary to it.
This occurs differently in the love of concupiscence and in the love of friendship. In the love of concupiscence, the one who desires something intensely moves against everything that is contrary to the attainment or peaceful enjoyment of what is loved. In this way, husbands are said to be jealous of their wives, lest the exclusivity they seek from them be impeded by the participation of others. In the same way also, those who seek excellence move against those who are considered excellent, as if the latter would hinder their own excellence. And this is the jealousy of envy, of which it is said in Psalm 36, “Do not be envious of the wicked, nor jealous of the criminal.”
The love of friendship seeks the good of the friend; therefore, when it is intense it leads a man to act against everything that is opposed to the good of the friend. And in this sense, someone is said to be jealous for his friend when he tries to reject everything that is said or done against the good of his friend. Thus, one is also said to be jealous for God’s glory when he tries to repel, according to his possibilities, what is against the honour or the will of God, according to what the book of Kings says: “I am consumed with jealousy for the Lord of hosts”; and regarding what the Gospel of John says: “The zeal for your house will devour me”, the Glossa comments that “he is devoured by good zeal who strives to correct any evil he sees; and, if he cannot, he tolerates it groaning”.
Cf. ST. THOMAS AQUINAS. Summa Theologica, I-II, q. 26, a. 4, co. transl. Aldo Vannucchi et al. São Paulo: Loyola, 2009, 2nd ed.
 According to medieval philosophy, the desire for pleasure generated by a physical, material reality.