When suffering from a severe illness, it can sometimes be difficult to see God's presence in our lives. The suffering we endure can make it seem like God has abandoned us.
However, often God wants to teach us something through the suffering, and to unite it to Jesus' suffering on the cross.
This makes it even more important to rejoice in thanksgiving to God when he restores us to health. By doing so we recognize God's providential hand and praise him for his merciful love.
Below is Psalm 116 that is sometimes labeled in Bibles, "Thanksgiving for recovery from an illness."