May He increase and make you abound in love means the same as: may He fill and overfill you with love. Saint Chrysostom expresses the meaning of both words with one: may He make it grow. Blessed Theodoret distinguishes the meaning of each word. He refers the first to the increase of the number of believers, and the second to the uniting of all of them in love.
"He prays that He may complete them in number and make them abound in love, that is, that they may acquire perfect love and have no lack in it at all."
In love toward one another and toward all. He wishes love exclusively because it is the bond of perfection (Col 3:14) and the fulfillment of the law (Rom 13:10). Christian progress is progress in love (1 Tim 6:11). Like the Lord Himself and the other Apostles, Saint Paul places love above all else in Christianity (1 Cor 13).
But whom does the Apostle mean by toward all? If by toward one another he means only the believers, the Thessalonians, then toward all will mean all other Christians. Thus Theodoret says: "That they may show love not only to one another, but also to all of the same faith, wherever they may be."
But if by toward one another we understand Christians in general, then toward all will mean all men. Thus Oecumenius says: "It belongs to Christians not only to love the brethren, but all men, both believers and unbelievers."
Saint Chrysostom understands love toward all, without specifying faith. "See how he wants to spread this love. Not only toward one another, but also toward all. For love according to God is truly revealed in this, that it strives to embrace all. But if you love one and not another, your love is human." The holy Apostle Paul often urges love toward all men. Thus Tit 3:2; 1 Tim 2:1; 1 Thess 5:15, and similar places.
As we do toward you. It is understood: are full and abound in love. The shortened expression requires supplemental thoughts. It may be understood thus: after speaking of love toward all, he also hinted at himself, as if saying: toward all, and toward us. After this, what follows naturally is: as we do toward you.
Or directly: love one another and all as we love you, that is, unselfishly and with readiness for every sacrifice, as he says of himself in chapter 2. He sets his own love, known to the Thessalonians and experienced by them, as an example. "Have us as the measure and pattern of love" (Theophylact).
It is customary for the holy Apostle Paul to set himself as an example (2 Thess 3:9; Phil 3:17). But he does not stop Christians at himself. From himself he raises them to Christ the Lord, saying: "Be imitators of me, as I am of Christ" (1 Cor 11:1). Thus he raises all to the One. In the work of moral education, example is stronger than any word.