"Joy" is in the Bible 155 times.  "Rejoice" is used 182 times.  

Nehemiah 8:10 makes it very clear that the Joy-of-the-Lord IS OUR STRENGTH.

God's children are to REJOICE and be JOYFUL in the Lord.

When we surrender our joy to worry, circumstances, anger, or people,

the enemy is able to attack, as he knows our "source of strength".