10 How, my brethren? When the farmer goes forth with the plough, carrying seed, is not the wind sometimes keen, and does not the shower sometimes deter him? He looks to the sky, sees it lowering, shivers with cold, nevertheless goes forth, and sows. For he fears lest while he is observing the foul weather, and awaiting sunshine, the time may pass away, and he may not find anything to reap. Put not off, my brethren; sow in wintry weather, sow good works, even while you weep; for, “They that sow in tears, shall reap in joy.”
They sow their seed, good will, and good works. “They went on their way and wept, casting their seed”. Why did they weep? Because they were among the miserable, and were themselves miserable. It is better, my brethren, that no man should be miserable, than that you should do alms....Nevertheless, as long as there are objects for its exercise, let us not fail amid those troubles to sow our seed. Although we sow in tears, yet shall we reap in joy. For in that resurrection of the dead, each man shall receive his own sheaves, that is, the produce of his seed, the crown of joys and of delight.
Then will there be a joyous triumph, when we shall laugh at death, wherein we groaned before: then shall they say to death, “O death, where is your strife? O death, where is your sting?” But why do they now rejoice? Because “they bring their sheaves with them.”
Source: The Enarrations, or Expositions, on the Psalms (New Advent)