Albert Barnes Commentary Psalms 41:3

"Jehovah will support him upon the couch of languishing: Thou makest all his bed in his sickness." — Psalms 41:3 (ASV)

The Lord will strengthen him upon the bed of languishing - The word rendered strengthen here means to support; to uphold; to sustain. The idea here is, that God would enable him to bear his sickness, or would impart strength - inward strength - when his body failed, or when but for this aid he must sink under his disease and die. The word rendered languishing means properly languor or sickness; and more generally something sickening; that is, something unclean, unwholesome, nauseating (Job 6:6). The idea here, in accordance with what is stated above, is, that acts of religion will tend to promote our welfare and happiness in this life; and more particularly that the man who shows favor (Psalms 41:1) to those who are weak, sick, helpless, will find in turn that God will support him when he is sick. Thus (Psalms 18:25), With the merciful you will show yourself merciful.

You will make all his bed in his sickness - Margin, as in Hebrew: “turn.” So the Septuagint, ἔστρεψας estrepsas. Luther renders it, “You help him.” The idea is, that God will turn his bed or his couch; that is, that he will render favor like turning his couch, or making his bed when he is sick; or, in other words, he will relieve his suffering, and make him comfortable on his bed. It does not mean that he will turn his sickness to health, but that he will relieve and comfort him, as one is relieved and soothed on a sick bed by having his bed made up.

This, too, is in accordance with the general sentiment that God will show himself merciful to those who are merciful, and kind to those who are kind. On the bed of languishing, it will be a source of great comfort to be able to remember that we, in our health, have contributed to the comfort of the sick and the dying.

1. The recollection itself will do much to impart inward satisfaction then, for we will then appreciate better than we did when we performed the act the value of this trait of character, and have a deeper sense of gratitude that we have been able to relieve the sufferings of others.
2. We may believe and trust that God will remember what we have done, and that he will manifest himself to us then as our gracious supporter and our comforter.

It will not be because by our own acts we have merited his favor, but because this is his gracious purpose, and because it is in accordance with his nature thus to bestow kindness on those who have been kind to others.