The right kind of sorrow is often needful and brings with it certain benefits. Yet, it is in the joy of the LORD that God's people specifically find strength.
God has a calling on each man’s life. Furthermore, He enables men to fulfill their appointed work. Man, however, must be strong and DO the work.
The Lord is the strength of His people. While it is true that we are called to do our part, God must add the necessary strength to accomplish the work.