Here is Zerr‘s comment on Pro 15.13, “A joyful heart makes the face cheerful, but by a painful heart the spirit is broken.”
“This verse is a statement of facts known to almost everyone. It may be asked why take up space in the Bible with something everyone knows. It is for the implied advice it contains. It should always be our desire to be pleasant in mingling with other people, and to that end we should cultivate a cheerful mind. If we imagine there is nothing about which to be cheerful, just “count your many blessings” and you may be surprised at what the Lord has done for you, and change your mind.”
NET Bible notes C.H Toy’s observation that “a cheerful face shows a courageous spirit.”
It takes courage in the Lord to have a joyful heart as one surveys the ruin that man has made of this present world.