We learn of the virtue of simcha in many places in the Torah. For instance:
“Sos Asis B’Hashem tagel nafshi b’Elokai (I will rejoice in Hashem, my soul will be joyful in Hashem)” (Yishayah 61:10)
We learn from the Navi that genuine happiness is brought upon a person who intimately connects to Hashem and calls Him Elokai (my Hashem). Living with such an intense bond enables him to live as though he has already been redeemed.
“Lev samaech yaiteev ge’ha… (A merry heart is good healing medicine…)” (Proverbs 17:22)
The Metzudat Dovid ztk”l explains that happiness restores the body like medicine, yet a broken spirit dries one’s mind and bones. Most of a person’s weaknesses come from emotional turmoil. When he comes to happiness, his face beams and serves as a remedy for his frailty.
Many studies show the impact that emotional health has on the physical body. Improvements in areas such as immune response, blood pressure and ability to ward off illness can be readily noticed. Nothing has such a direct effect to ruin health and waste life more than grief, worry and a bad temper- in case we needed to reinforce more reason to put on a smile.