19 “Hearken unto me, O Lord, for sweet is Your mercy”. He has given this as a reason why He ought to be hearkened unto, because sweet is the mercy of God....To a man set in trouble the mercy of God must needs be sweet. Concerning this sweetness of the mercy of God see ye what in another place the Scripture says: “Like rain in drought, so beautiful is the mercy of God in trouble.” That which there he says to be “beautiful,” the same he says here to be “sweet.” Not even bread would be sweet, unless hunger had preceded.
Therefore even when the Lord permits or causes us to be in any trouble, even then He is merciful: for He does not withdraw nourishment, but stirs up longing. Accordingly what says he now, “Hearken to me, O Lord, for sweet is Your mercy”? Now do not defer hearkening, in so great trouble I am, that sweet to me is Your mercy. For to this end You deferred to succour, in order that to me that wherewith You succoured might be sweet: but now no longer is there cause why You must defer; my trouble has arrived at the appointed measure of distress, let Your mercy come to do the work of goodness. “After the multitude of Your pities have regard unto me:” not after the multitude of my sins.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)