"A merry heart makes a cheerful countenance,
.But by sorrow of the heart the spirit is broken."
. . . . . . . . . . . . . . . . . . . . (Proverbs 15:13)
What is the true meaning of the verse?
I see this means that what is in your heart will effect what is showing on your face. And Jesus has said out of the abundance of the heart the mouth will speak > Luke 6:45. So, what is really in our character will come out, even physically.
But, also > what is in us will effect our spirit.
So, a point is > if you are trusting God and being thankful to Him, this will be good for you, even in your body. But if we give in to negative things and let things of this life get the better of us > this helps keep us weak so our spirit can be broken. Jesus warns how people can be choked by cares of this life > there are those who don't receive and live God's word and they are "choked with cares, riches, pleasures of life, and bring no fruit to maturity." (in Luke 8:14)
These are choked by thorny plants. There are thorns in the evil spirit who has people under the power of money and pleasures and concerns of this life > Paul says, "I will not be brought under the power of any," in 1 Corinthians 6:12. We can care about things, but stay with God and how He has His way with us and decides how we are and what we do > Colossians 3:15.
Does it literally mean you have to walk around with a smile on your face at all times?
Well, I consider it is talking about how things work out from our hearts > our real character . . . not only our acting.
Being an Introvert curiosity is there.
As we grow in Jesus, we discover how our personality develops. Jesus is "gentle and lowly in heart, and you will find rest for your souls." (in Matthew 11:28-30)
But, yes, everything in God's word is talking about you, somehow

Every scripture can somehow be used to help us find out how to love. About being an introvert > it is good to be feeling and thinking about things, but also it is good to be expressive > whatever is good at any time > maybe we could say, to first be prayerful and hear things out and evaluate, then express what is good. In God's love we are creative in our evaluation and in our expression
