Why do they sow amidst tears?
Rashi: People who sow in a dry land, they worry and think that it will not sprout. They reap amidst song, via the springs.
Radak: He supplicates to Hashem to send rain, so he will reap what he sowed with a Brachah. It is very unlikely that he will reap if Hashem will not have mercy. So Yisrael in Galus, amidst all their afflictions, they sow (do Mitzvos) amidst tears, and hope that Hashem will take them out of Galus.
Malbim: He already despaired of Peros, that there will be any result from his toil.
Why do they reap amidst song?
Rashi: This is via the springs. Malbim - suddenly springs came and watered the land. At the time of harvest his mourning is converted to Simchah; he never hoped for this.
Radak: Reaping is the reward [for the Mitzvos that they did].
