Jamieson, Fausset and Brown on 1 Thessalonians 3:9

9. 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," 1Th 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 1Th 2:20; Joh 15:11).