Matthew 9:15 English Standard Version (ESV) And Jesus said to 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.
Beyond the general concept of Jesus being the bridegroom here, there is a challenge to the practices being observed by those who worship God in fasting. Since these new disciples need not fast as long as Jesus (the Messiah) is present with them, why do those who worship God fast? What is there understanding of fasting? Jesus is challenging their understanding and their practice.
“Many customs, purely secular in their origin, have gradually obtained a religious significance, just as purely religious customs have been dissociated from religion. It is also possible and, in the light of some usages, probable, that different motives operated in the association of fasting, as of some other customs, with religion. Scholars have been too ready to assume that the original significance of fasting was the same in all countries and among all nations. Robertson Smith in his Religion of the Semites advanced and defended theory that fasting was merely a mode of preparation for the tribal meal in which sacrifice originated, and came to be considered at a later stage as part of the sacrificial act. This hypothesis apparently accounts for the otherwise strange fact that both fasting and feasting are religious acts, but it does not give a satisfactory explanation of the constant association of fasting with the “wearing of sackcloth,” the “putting of ashes on the head,” and other similar customs. It is obvious that very different motives operated in the institution of fasting and of feasting religious observances.” Ref. quoted from International Standard bible Encyclopedia
Leviticus 16:29 offers a statute in which Israel was told to afflict themselves before the atonement sacrifice was made. That affliction was defined as humbling oneself and over time has been used by religious leaders to call for a fast, to deny the flesh. Note the difference between the statute and the practice.