Why are miserable Christians said to be out of fellowship with the Lord?

Miserable Christians are described as out of fellowship with the Lord because they are living in the world where they should not be. God does not want ignorant, ungrounded believers but wants His children well taught, instructed, grounded, and enjoying a joyous faith.

Les explains from Colossians 3 that God does not want a bunch of ignorant believers in the Body. He wants every child of His to be well taught, well instructed, grounded, and to enjoy their faith. He therefore says there is no room for miserable Christians. If they are miserable, they are out of fellowship with the Lord and out in the world where they should not be. But the Christian life is a joyous life, and God wants it to be that way, with His peace ruling in their hearts and the Word of Christ dwelling in them richly.

Colossians 3:15

And let the peace of God rule in your hearts, to the which also ye are called in one body; and be ye thankful.

Why is a Christian miserable, and what does that reveal about their fellowship with the Lord?

A Christian is miserable when they are out of fellowship with the Lord and have drifted into the world where they should not be. This reveals a broken fellowship, not God's intention, since He wants believers well taught, grounded in His Word, and enjoying a joyous life.