The joy of the Lord in you is your strength. Without the joy that is bubbling or rises in your spirit,the strength which means spiritual energy will not be effectively in you.
That is why,one of the strategies your enemy uses is to attack your joy. Once,he/she can successfully knock your joy out of you and get you gloomy and unhappy,it will be difficult for your faith to work and prophecies results.
Thus,the joy of the Lord is your strength. .