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.
Solomon's mother advised him not to give his strength unto women. He ignored this advice and his wives turned away his heart after other gods.
Man must spend his strength on things that matter. In doing so, we must focus our efforts upon the right things while working through and by God's power.
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.
