Thomas Aquinas Commentary Lamentations 3:31-33

"For the Lord will not cast off for ever. For though he cause grief, yet will he have compassion according to the multitude of his lovingkindnesses. For he doth not afflict willingly, nor grieve the children of men." — Lamentations 3:31-33 (ASV)

Here he shows the divine mercy regarding the relaxation of punishment.

First, he presents the release from punishments: the Lord will not cast off, because He will not always punish. The Lord will not cast off his people (Psalms 94:14). The thresher shall not thresh it forever, neither shall the cart wheel hurt it (Isaiah 28:28).

Second, he makes an argument from the divine tenderness: for if he has cast off, he will also have mercy, for it is characteristic of a tender father to offer consolation after striking for the sake of correction. If it be under correction, it shall be allowed to come to your mercy . And from His love for humanity: for he has not humbled from his heart, that is, driven by His love. The children of men shall put their trust under the covert of your wings (Psalms 36:7).