5 Then he goes on: Like a weaned child upon his mother's breast, so will you reward my soul. We are told that when Isaac was weaned Abraham made a feast because now that he was weaned he was on the verge of boyhood and was passing beyond milk food. The Apostle feeds all that are imperfect in the faith and still babes in the things of God with the milk of knowledge. Thus to cease to need milk marks the greatest possible advance. Abraham proclaimed by a joyful feast that his son had come to stronger meat, and the Apostle refuses bread to the carnal-minded and those that are babes in Christ. And so the Prophet prays that God, because he has not lifted up his heart, nor walked amid things great and wonderful that are above him, because he has not been humble-minded but did lift up his soul, may reward his soul, lying like a weaned child upon his mother: that is to say that he may be deemed worthy of the reward of the perfect, heavenly and living bread, on the ground that by reason of his works already recorded he has now passed beyond the stage of milk.
Source: Homilies on the Psalms (New Advent)