Jamieson, Fausset and Brown on 1 Thessalonians 4:9

9. brotherly love, &c.--referring here to acts of brotherly kindness in relieving distressed brethren. Some oldest manuscripts support English Version reading, "YE have"; others, and those the weightiest, read, "WE have." We need not write, as ye yourselves are taught, and that by God: namely, in the heart by the Holy Spirit (Joh 6:45; He 8:11; 1Jo 2:20, 27).

to love--Greek, "with a view to," or "to the end of your loving one another." Divine teachings have their confluence in love [Bengel].