Today's gospel is Matthew 9:14-15. It helps me to look at the previous few verses as well.
Matthew 9:10-13
10 While he was at table in his house, many tax collectors and sinners came and sat with Jesus and his disciples.
11 The Pharisees saw this and said to his disciples, "Why does your teacher eat with tax collectors and sinners?"
12 He heard this and said, "Those who are well do not need a physician, but the sick do.
13 Go and learn the meaning of the words, 'I desire mercy, not sacrifice.' I did not come to call the righteous but sinners."
Matthew 9:14-15 -- Today's Reading
14 Then the disciples of John approached him and said, "Why do we and the Pharisees fast (much), but your disciples do not fast?"
15 Jesus answered them, "Can the wedding guests mourn as long as the bridegroom is with them? The days will come when the bridegroom is taken away from them, and then they will fast.
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Jesus was a walking contradiction to the Pharisees. The Pharisees look bad to us today, but I personally believe they meant well, or at least started out that way. They struggled to live to the letter of what they perceived the law meant, and in doing that they separated themselves from sinners. They felt they needed to act pious, and set a very public example of how the Law was to be followed.
Jesus turned this all on its head. He walked in and sat down with tax collectors and sinners, and in verses 12-13 you can see his reasoning. In v14-15, we see that the religious of the time just didn't get it. Here they were fasting and sacrificing, and Jesus' disciples did none of that. Christ's answer is so poignant and powerful. I'm not going to be here forever, we're not going to waste our time with such things while I'm here. Soon enough I will be gone, and they can go back to those spiritual disciplines.