לזכות יהודי השרוי במצוקה - שיזכה לשיר שירת השם
The primary task of a person in this world is to sing the praises of Hashem! The dissatisfaction that people have from life stems from the fact that they are not singing to Hashem on a constant basis. It is not that we are dissatisfied and therefore don't sing but on the contrary - we don't sing and are therefore dissatisfied.
We lost our power to sing after the sin of Adam Harishon. When we were leaving Egypt we were accompanied by Paroh and were unable to sing. Once we entered the sea, the place that the sin of Adam had not blemished - we were able to sing the great shiras ha-yom.
One day soon we will see the fulfillment of the pasuk ומלאה הארץ דעה את השם כמים לים מכסים - The earth will be filled with knowledge of Hashem like the water covers the sea. We will be in a place of the "sea" where there is no sin and the ability to sing is complete. The pasuk אז ירננו כל עצי היער - All of the trees of the forest will sing to Hashem.
Sadness emanates from a lack of song. Song is life and life is song.