This is why:
A merry heart makes a cheerful countenance: but by
sorrow of the heart the spirit is broken. Prov 15:13
All the days of the afflicted are evil: but he that
is of a merry heart has a continual feast. Prov 15:15
A merry (cheerful) heart does good like a medicine:
but a broken spirit dries the bones. Prov 17:22