We see in the readings today the transformation of religion from an attempt to appease God with gifts to a system of obedience. Is religion primarily about God or about us? The old religion thought that the problem was an angry God. In this manner of thinking humanity would get along just fine if God would leave us alone, so the primary task of the religious person is to make the angry God happy. In the new religious sense that these readings teach, the problem is us human beings. Therefore, the new religion prizes obedience over sacrifice of animals.
This transformation of religion goes on throughout the Old Testament, and indeed is still going on in our hearts. Even today we have a tendency to think of God like a senile relative who should be respected but mostly calmed down. We hope that if we go to Mass on Sunday and say our prayers and even go to Mass during the week, maybe God will be satisfied and we can live the rest of our lives without any interference.
However, we ought not think this way, and we know it. God is not getting in our way; we are getting in his way. He is trying to save us, so that we can live with him forever in heaven, but we are not being very cooperative. We are the ones who should be learning from God. We are the ones who are frustrating to work with. If only we would be obedient to his commands everything would be perfect.
The Pharisees in the Gospel see fasting in that same way that Saul saw sacrifice: as a gift they are giving to God, as a way of appeasing God's anger. Jesus' point is that fasting is supposed to be a reminder for us that we are separated from God. The Apostles cannot fast because while Jesus is with them they are not separated from God.
Our separation from God, which humanity has experienced ever since the Fall, ought to make us sad; it ought to make us feel an intense longing, but sin prevents us from seeing this clearly. Fasting focuses our attention: we feel physical hunger, and that symbolizes the spiritual hunger that we fail to recognize. Fasting, obedience, and everything else that God asks of us are not his whims or his selfish desires, but exactly the remedy that our souls require.