"...every day with Jesus is not sweeter than the day before. Some days with Jesus our disposition is sour. Some days with Jesus we are so sad we feel our heart will break open. Some days with Jesus we are so depressed and discouraged that between the garage and the house we just want to sit down on the grass and cry...
For David says in Psalm 19:7, "The law of the LORD is perfect, reviving the soul." If every day with Jesus were sweeter than the day before, if life were a steady ascent with no dips in our affection for God, we wouldn't need to be re-revived...
Our joy is not static. It fluctuates with real life. It is vulnerable to Satan's attacks...
But the Holy Spirit has given us a sword called the Word of God for the defense of our joy."

from Desiring God by John Piper