The heart of homesickness is this deep longing for someplace or someone. It runs so deep that it dominates one's thoughts and attention. Psalm 73:25-26 states
"Whom have I in heaven but You? And besides You, I desire nothing on earth.
My flesh and my heart may fail, But God is the strength of my heart
and my portion forever."
Whenever I read that I think - REALLY? Can I have that kind of single-heartedness - that kind of desire for God. It is what John Piper calls a "homesickness for God." It is interesting that the Psalmist declares this when his flesh is failing. The truth is, I sometimes do have that hunger and heart for the Lord. It is also true that sometimes my heart is dulled to Him, "used" to His presence, unappreciative of His gifts. In Piper's outstanding book on fasting - "A Hunger for God" - he suggests that our longing and homesickness for God is because we nibble so much at the table of the world - we are not hungry for the Lord anymore. In Deuteronomy 8, the Lord tells the people that He let them be hungry to show what was in their hearts and to lead them to understand that "man does not live on bread alone." He uses hunger to enliven their hearts to Himself. Later in the chapter - after telling them to enjoy the food and fatness of the land - He warns that those same good delights can cause dullness and a loss of hunger for God.
It seems we go through cycles of being alive to the Lord - desiring Him above all things - and then we get filled up on other things - even good things - and our hunger for the Lord is stilled. So what is the solution. There are perhaps many... I think the regularity of Sabbath (ceasing and resting) is one practice that reorients our heart. Another practice is fasting - to come before the Lord for one meal or for days - and say - "a renewed hunger for You, Lord is more important than food - more important than life!" Somehow - in fasting - we are reshaped, awakened to His good gifts, filled with gratitude and prepared for His callings. I am reminded in fasting how good all of God's great gifts taste.