I totally agree with you that it was not some divine payback.  But we believe that Jesus is fully divine, and fully human, both God and man.  As such His divinity yielded full to God's Will easily, I believe, because He is the begotten Son of God, He is God, the Second Person of God.  That being said, He is also fully man...the human son of Mary, the Virgin Mother.  As such, He agonized in His human self in prayer in the Agony in Garden, even to the point of sweating drops of blood. His human nature struggling, but submitting to what He knew God was asking of Him.  (Luke 22, The Agony in the Garden.) Jesus in His Divine nature doesn't do this, for me, in His human nature He struggles and submits to this. Why?  Like you said, not because He needed to do as some sort of "payback".I prefer to see it as profound message to us humans as to what may need to submitted in our individual lives if we decide to follow Him.  This serious commitment  we have seen in the lives of many holy ones.