The Joy of the Lord is our strength for the body.

"A merry heart doeth good like a medicine." This is the divine prescription for a weak body. And so on the other hand, despondency and depression of spirits are the cause of nervousness, head-ache, heart-break, and low physical vitality. A word of cheer and an impulse of hope and gladness will often break the power of disease. 

It is true there is a deeper cause and a diviner power than the mere natural influence of joy. Incurable disease can only yield to the actual touch of Divine omnipotence, but joy is the channel through which the healing waters flow, and the overflow of the life of Christ in both soul and body.  If you would live above your physical conditions, if you would renew your strength continually and "mount up on wings as eagles, and run and not be weary, and walk and not faint," if you would carry in your veins the exhilaration and zest of unwearied youth and freshness, if you would know, even here, in all its fullness, the foretaste of the resurrection life in your body, if you would be armed against the devil's shafts of infirmity and pain, and throw off his arrows upon your body as the heated iron repels the water which will not lie upon it, then, beloved, "Rejoice in the Lord always, and. again I say, Rejoice."