“I have inclined my heart to the doing of your just commands, forever, for the reward” (Ps 118[119:112). The Psalmist who here says, “I have inclined my heart,” earlier had said, “Incline my heart,” so that we would understand that this is at once a divine gift and the work of our own will. But does this mean that we will be doing God’s just commands for all eternity? The deeds we perform to meet our neighbors’ needs cannot be eternal, since their needs are not. If we do not now perform those deeds out of love, there is no justification. If we do them out of love, that love is eternal, and for it an eternal reward has been prepared. It’s for the sake of that reward that he says that he has inclined his heart to the doing of God’s just commands so that, eternally loving, he might deserve to possess what he loves forever. (EnPs 118/23, 8; PL 37, 1569)
So, it seems, the reward is being able to love God forever. Eternal fulfilment in eternal possession of what we love.