The hymn by Henry Van Dyke beautifully expresses the importance of having a merry heart:
Joyful, Joyful, we adore Thee,
God of glory, Lord of love,
Hearts unfold like flowers before Thee,
Opening to the sun above.
Melt the clouds of sin and sadness,
Drive the dark of doubt away;
Giver of immortal gladness,
Fill us with the light of day.
This hymn declares that God is our ultimate source of joy. The Scriptures agree: “the fruit of the Spirit is. . . joy” (Gal 5:22), and “the joy of the Lord is your strength” (Neh 8:10). The life of abiding in Christ should bear the fruit of joy continually. Jesus declared “These things I have spoken to you, that My joy may remain in you, and that your joy may be full.” (John 15:11).
1. Since Christ is our source of wisdom, He enables us to live above our discouraging circumstances.
“A merry heart does good, like medicine,
but a broken spirit dries the bones” (Prov. 17:22).
This quality of life does not exempt us from valid emotions such as sorrow and grief. For instance, the apostle Paul’s team could admit their feelings, yet be sustained by the supernatural joy of their indwelling Savior: “as sorrowful, yet always rejoicing; as poor, yet making many rich; as having nothing, and yet possessing all things” (2 Cor. 6:10). This divine grace of joy is available to every child of God in an unlimited supply. Yet we are responsible to keep abiding in Christ, thereby maintaining unhindered fellowship with our Source of joy.
2. Another requirement for experiencing joy is to consciously “set our minds” on the infinite blessings which belong to the true believer in Christ.
“Set your mind on things above, not on things on the earth. For you died, and your life is hidden with Christ in God.” (Col 3:2,3). If we are encumbered by problems such as guilt (Psalm 51:7-11), worry (Phil. 4:4-7), or ingratitude (1 Thess. 5:16-18), joy will trickle out of our heart like water through a sieve. On the other hand, if we focus on our blessings–such as our personal salvation, spiritual identity, and hope–then, we can “rejoice evermore” (See Deut. 16:15; Luke 10:20; John 4:36; 3 Jn. 3,4; John 8:31,32; Rom. 12:12).
F.J.Huegel points the way to grasping the implications of letting Christ live through us:
“Oh, the unutterable glory of this mystic union with Christ! … The faintest glimmer of light from His holy countenance, beside which the sun is but a shadow, suffices the heart. Paul could not have done otherwise than sing there in prison when at midnight with Silas, he rejoiced. His back was bleeding with stripes; it was midnight; he was in prison. But he sings as a participant of Christ; heaven’s ineffable light bursts in about him, and he forgets all but he fact of Christ.” 
With Christ as our life, let us rejoice evermore!
Our blessed God, we rejoice that You have given us every spiritual blessing in Christ. We gladly abide in Christ so that His joy may be overflowing in our hearts. In Your name, amen.
 F.J. Huegel, Bone of His Bone p. 83.
Copyright, 1998, 2013 by John Woodward. Permission is granted to reprint this article for non-commercial use if credit is given to the author and GraceNotebook.com. Biblical quotations are from the NKJV (Copyright, Thomas Nelson).