The seed, the source, the essence of unhappiness
(James Smith, "The Way of Salvation Set Forth")

Sin is the seed, the source, the essence of unhappiness. "There is no peace to the wicked," nor is there any happiness for the sinner in his sins. The carnal mind may find something like pleasure in carnal things — but real, solid, lasting joy, cannot be found, never has been found by the sinner, until converted to God. Carnal pleasure is empty, fleeting, and unsatisfactory in its very nature.

The true Christian has enough to make him genuinely happy!
All of his sins are forgiven and forgotten.
He is adopted into God's family.
His person is justified before God.
He is clothed in the perfect righteousness of Jesus.
He is a child of God.
He is regenerated by the Holy Spirit.
He has the Spirit of adoption in his heart.
He is at peace with God, and God is at peace with him.

All the promises of God are his, which represent Jehovah as pledged to support, sustain, comfort, supply, and bless him in time and eternity! His God is with him. His God is for him. His God will never fail him in any trial, nor leave him under any circumstances for one moment.

Such is the Christian's blessed state, and such are his blessed privileges — though he may appear poor, afflicted, and despised among men.

In proportion . . .
  as sin is subdued,
  as sanctification is deepened,
  as the Savior is prized,
  as our talents are laid out for the Lord's glory
 — are we happy. But if sin is allowed to conquer, if personal sanctification is neglected, if the intimations of the Holy Spirit are slighted — then the believer is not, and cannot be happy.

Let the Christian therefore . . .
  cleave unto the Lord with full purpose of heart,
  cultivate close fellowship with God,
  walk softly, uprightly, and daily with God —
so will his peace be like a river, and his path be like the shining light which shines more and more unto the perfect day.

"You will keep him in perfect peace, whose mind is stayed on You, because he trusts in You!" Isaiah 26:3