Proverbs 15:13 A merry heart maketh a cheerful countenance: but by sorrow of the heart the spirit is broken.
This is a truism that we all know, but it is hard to remember how to apply this truth in real life. What’s on the inside comes out through the eyes and facial expressions, even though we may try to be actors and paste on a smiley face. Outward attempts to make a joyful face and a happy smile do not work. It’s not genuine,
It must work the other way around, that is, work on the heart, and the internal spirit will affect the face and produce a cheerful countenance. That’s the promise: a happy inside makes a cheerful outside. How does one work on the heart? You don’t; you can’t. God does. The Holy Spirit brings to faith through the gospel, and then He sanctifies the believer by changing the heart and making it merry. Then the joyful heart makes a happy face. All genuine work on the person begins with the heart, which positively affects the behavior and appearance of the body.
The sorrowful soul cannot just “cheer up.” The spirit is broken by sorrow, usually caused by sin. Into that breach Sin and Satan enter and bring the soul down. The forgiveness of sins and the life enter by faith in the Gospel; the Spirit fixes the brokenness and sorrow and makes the heart merry. The renewed inner man makes the makes outward man smile and rejoice.