The Pharisees, and others in Israel, expected the coming Messiah to crush the wicked and save the righteous. Indeed, Jesus the Messiah will do just that at His second advent, but the mission of His first coming was to seek and to save that which was lost, that is to save His people from their sins (2Thess 1:3-10; Luke 19:10; Mat 1:21). The self-righteous Pharisees not only lacked the mercy Jesus showed to tax collectors and sinners, but despised them (Luke 18:9-14). They did keep the law outwardly. They offered the prescribed sacrifices, they gave tithes of everything they owned including their spices of mint, and dill and cumin, and they fasted on the prescribed Day of Atonement and added other fast days not commanded in the law. When they questioned Jesus on the propriety of eating with sinners, He quoted Hosea 6:6, and urged them to learn what it meant. In essence, God requires loyal love, and that it be shown to others, including sinners (Mat 9:9-13).
The Pharisees rebuked Jesus because He ate with sinners, and also because His disciples did not fast (9:14-17). Fasting is only commanded once in the Bible, and that under the law of Moses as a means of humbling oneself on the yearly day of atonement (Lev.16). Fasting is here associated with mourning a loss, but since Jesus was still with them, His disciples need not mourn and fast. Jesus had ushered in a new age in which things like fasting and sacrifices would become old and obsolete. In the same way that a new patch must not be sewn onto an old garment or new wine poured into old wineskins, the new practices of this age must not be added to the old ones.