The state of our hearts is vitally important to the way that we live - and whether we will live in joy or sadness. Also, as we will learn from this passage, the state of our countenance is also something about which we should be concerned. To say that we are Christians and know God's joy, yet for this joy never to reach our face (i.e. our countenance) is a bad testimony to those around us - and especially to the lost. This proverb holds a great deal of wisdom for us if we will open it and learn from it.
First, we have the joyful heart. This heart is the one filled with joy and as a result is exceedingly glad. This gladness does not come from mere human prosperity - but truly comes from and is maintained by the blessings of the Lord. In the New Testament this joy comes from knowing Christ Jesus and the salvation that He brings to us. It comes from knowing that God loves us and that we are saved from the wrath of God through Him. This is a joy that floods our hearts no matter what our outward circumstances. It is a joy, as Peter expresses it, that causes us to greatly rejoice with joy inexpressible and full of glory. The foundation of this joy - obtaining the outcome of our faith - which is the salvation of our souls (1 Peter 1:8-9). This joyful heart communicates with our countenance - and brings us to have a cheerful face. No matter our circumstances we can stop and remember that our sins are washed away - that we are made righteous in Christ - and that as a result - we are saved. That can bring a cheerful expression to any face.
I find it disturbing that some are what I would call, "lemon-sucking Christians." They seldom have a joyful or cheerful expression on their face. They always seem sour about something. It is almost that they are unwilling to be happy and joyful. This is a horrible witness to Christ and to the salvation that He brings. Sure there are things about which I am concerned - even heartbroken. But, in spite of everything that happens and can happen - the one constant is my salvation - that I will not face the wrath of God and I will enjoy fellowship with God here and now - and forever in heaven.
Then there is the sad heart. The word used for "sad" here is "assebet" which clearly refers to emotional suffering - and not usually to pain or injury. This is a sadness of heart. It is truly fascinating to look at what brings on "assebet" in someone Scripturally. This kind of sadness or grief is caused by idolatry (Psalm 16:4), by a fear of spiritual discipline over our sin (Job 9:28), or by those who "wink the eye" in evil plots (Prov. 10:10). Some might refer to this to speak of someone with a broken heart over sin or over the loss of a loved one - but that is not the way that "assebet" is used Scripturally. This is a sadness brought on by disobedience, sinfulness, and a walk contrary to the ways of God. This kind of sadness causes our spirits to be broken. The word "broken" here means that we are stricken, smited, or scourged. We have a bad situation in our lives due to sin - and our spirits are broken due to the consequences of our sin.
When you look at this proverb and the meanings of the words used in it, you see that wisdom therefore is to walk with God. When we do joy will come to us - and our countenance, our face will reflect it. But to walk in disobedience and rebellion is to invite a brokenness and sadness into our lives which will fill our hearts with emotional suffering. How many in our world walk in this kind of suffering every day. But, dear saints of God, we can alleviate this suffering by sharing the gospel with them and encouraging them to come to Christ - Who can deliver them from their sin and flood their hearts with His own joy. This Proverb truly helps us to see the difference between the lost and the saved - between those who embrace obedience to God versus those who mock such a lifestyle. The difference is the Source of their joy. The difference between a sad countenance - and a cheerful one - is the gospel and the joy of knowing that we are saved.