Joy is one of the best safeguards against temptation. The devil is a carrier of dust and dirt and he uses every opportunity to throw what he has at us. But a joyful heart protects us from this dirt. That is because Jesus is there in our joy. Jesus takes full possession of our soul when we surrender to him joyfully.
This picture of my grandmother just resonates joy to me. Even though her mind is ravaged by a terrible disease, Jesus is in her heart. She has mostly forgotten me, but hasn't forgotten her Savior. Sure, she will have her moments where her confusion and emotions get the best of her, but if you compare her to others with a similar diagnosis, you will see a difference. That difference is the joy from within, placed there by Jesus Himself. And nothing, no matter how terrible it may be, can take that away.