You, God, are my God, earnestly I seek you; I thirst for you, my whole being longs for you, in a dry and parched land where there is no water.
David affirms God as his God. Not just the God of his people or ancestors but his own God. He seeks him earnestly, not just looking for him in random prayers or fellowship. David was intentional in growing closer to HIM. He seeked Him seriously, purely.
Other versions call the entire being the soul.
In the deepest corners of our souls, God must reign. He must be king in our minds, emotions, will.
Minds – Finally, brothers and sisters, whatever is true, whatever is noble, whatever is right, whatever is pure, whatever is lovely, whatever is admirable—if anything is excellent or praiseworthy—think about such things.
Philippians 4:8 NIV
Emotions – He will yet fill your mouth with laughter and your lips with shouts of joy.
Job 8:21 NIV
David’s soul and body thirst for God to the point of death. The soul is comprised of mind, will and emotions. His mind/thoughts desires the Lord alone. His will desires to please God alone. His emotions long to feel God alone. Since extreme thirst is not just physical but also emotional, he not only needs God, he also wants Him to quench his thirst and take away the emotional agony of being extremely thirsty. It means needing and desiring God as if our lives depend on it, and it actually does.