“And Jesus said unto them, Can the children of the bridechamber fast, while the bridegroom is with them? as long as they have the bridegroom with them, they cannot fast.”
Jesus answered, how can the guests of the bridegroom fast while he is with them? They cannot, so long as they have him with them — the bridegroom metaphor is the most Christological claim in this chapter after the Son of Man saying. In the Old Testament, God is frequently the bridegroom of Israel (Isaiah 62:5, Hosea 2:19–20, Ezekiel 16); here Jesus implicitly identifies himself as the bridegroom. The wedding is the occasion that makes fasting inappropriate — you do not mourn at a wedding. Jesus is present, the wedding has begun, the kingdom has arrived: this is the time for celebration, not for the fasting that expressed mourning over the exile and the divine absence. The not while he is with them anticipates the time when he will be taken away.
Church Fathers on Mark 2:19
For the disciples of John being in an imperfect state, continued in Jewish customs.
But it may be thought that He added Pharisees, because they joined with the disciples of John in saying this to the Lord, whilst Matthew relates that the disciples of John alone said it: but the words which follow father shew that those who said it spoke not of themselves, but of others.
But John did not drink wine and strong drink, because he who has no power by nature, obtains more merit by abstinence. But why should the Lord, to whom it naturally belonged to forgive sins, shun those whom he could make more pure, than those who fast? But Christ also fasted, lest He should break the precept, 'He ate with sinners,' that thou mightest see His grace, and acknowledge His power.