A.T. Robertson Commentary 1 Thessalonians 3:9

"For what thanksgiving can we render again unto God for you, for all the joy wherewith we joy for your sakes before our God;" — 1 Thessalonians 3:9 (ASV)

Render again unto God (τω θεω ανταποδουνα). Second aorist active infinitive of double compound verb αντ-αποδιδωμ, to give back (απο) in return for (αντ). Old verb rare in N.T., but again in 2 Thessalonians 1:6.

For you (περ υμων). Around (concerning) you, while in verse 2 υπερ (over is used for "concerning your faith."

For (επ). Basis again as cause or ground for the joy.

Wherewith we joy (η χαιρομεν). Probably cognate accusative ην with χαιρομεν attracted to locative χαρα (Matthew 2:10).