God is in control.
This Psalm is an overdose of hope. What David basically says here is: "My heart is glad, I am at ease, and my body rests secure." Why does David have this assurance? David is completely certain that God will never abandon him, neither in life nor in death.
Let us remember that in God's presence there is fullness of joy. Here he is NOT talking about "a little joy," he is talking about FULLNESS, the ultimate joy, and this is being with God.
If you are going through difficult times today, remember that God is in control. Your body and soul can know that you are not alone. In God we can find direction in life. God will guide us on the right path. In God we have EVERYTHING we need!
Therefore my heart is glad, and my soul rejoices;
My flesh also will lie down safely.
10 For you will not abandon my soul to Sheol,
Nor will you allow your Holy One to see corruption.
11 You will show me the path of life;
In your presence there is fullness of joy;
Pleasures are at your right hand forever.
Psalm 16:9-11