4 “The Lord help him”. But when? Haply in heaven, haply in the life eternal, that so it remain to worship the devil for earthly needs, for the necessities of this life. Far be it! You have “promise of the life that now is, and of that which is to come.” He came unto you on earth, by Whom were made heaven and earth. Consider then what He says, “The Lord help him, on his bed of pain.” The bed of pain is the infirmity of the flesh; lest you should say, I cannot hold, and carry, and tie up my flesh; you are aided that you may.
The Lord help you on your bed of pain. Your bed did carry you, you carried not your bed, but wast a paralytic inwardly; He comes who says to you, “Take up your bed, and go your way into your house.” “The Lord help him on his bed of pain.” Then to the Lord Himself He turns, as though it were asked, Why then, since the Lord helps us, suffer we such great ills in this life, such great scandals, such great labours, such disquiet from the flesh and the world? He turns to God, and as though explaining to us the counsel of His healing, He says, “You have turned all his bed in his infirmity.”
By the bed is understood anything earthly. Every soul that is infirm in this life seeks for itself somewhat whereon to rest, because intensity of labour, and of the soul extended toward God, it can hardly endure perpetually, somewhat it seeks on earth whereon to rest, and in a manner with a kind of pausing to recline, as are those things which innocent ones love....The innocent man rests in his house, his family, his wife, his children; in his poverty, his little farm, his orchard planted with his own hand, in some building fabricated with his own study; in these rest the innocent.
But yet God willing us not to have love but of life eternal, even with these, though innocent delights, mixes bitterness, that even in these we may suffer tribulation, and so He turns all our bed in our infirmity. “You have turned all his bed in his infirmity.” Let him not then complain, when in these things which he has innocently, he suffers some tribulations. He is taught to love the better, by the bitterness of the worse; lest going a traveller to his country, he choose the inn instead of his own home.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)