Jamieson, Fausset & Brown

For what thanks--what sufficient thanks? render . . . again--in return for His goodness (Psa 116:12). for you--"concerning you." for all the joy--on account of all the joy. It was "comfort," Th1 3:7, now it is more, namely, joy. for your sakes--on your account. before our God--It is a joy which will bear God's searching eye: a joy as in the presence of God, not self-seeking, but disinterested, sincere, and spiritual (compare Th1 2:20; Joh 15:11).