The deep, deep joy of The Soldier of The Cross comes from the unbridled Presence of the Indwelling Holy Spirit.
How does this happen? By obedience to God's Word.
Joy is not something that comes like a bolt of lightening, nor is it a feeling that is the result of inspiration.
Joy is the result of the absence of sin and the organic Presence of God in the life of a Servant who is determined to be a Living Sacrifice.
Where does the song in the heart come from? Is it true that the stars sang before the earth was created? The music of the soul is the result of the union between God and man that is brought about by man's acceptance of the gift of Salvation.
Salvation is experienced daily by those who obey His Word and the result is protection from all of the unhealth of disobedience.
Soldier, the deeper, the simpler. He has not made it hard. He is clear. We do this and He will do that. Most of God's Promises are conditional, based on obedience.
Is there a song in your heart today? Even those who have experienced the most difficult pain and loss will say they have known his comfort and joy because of agreement with God.
If you have many years or only a few days, it is all the same. You must let Him have the day and the night. Your thoughts must agree with His Word and your mouth and actions must reflect that agreement.
For this you were created, and anything else is simply foolish and a waste of limited days.
Look up and move on!