“The disciples of John approached him and said, "Why do we and the Pharisees fast (much), but your disciples do not fast?” (Matthew 9:14)

According to the notation of Studium Biblicum: Fasting in the Bible is an expression of humility and inferiority, meaning obedience and respect towards God. It depends whether abstaining from some or all food and drink. The Law of Moses demands fasting on the Day ofAtonement, that is the tenth of July ever year.
After the exile, there is one day fasting on April, May, July and October every year, to commemorate the depredation of the Temple. The Pharisee fasted every Monday and Thursday, while the disciples of John fasted very often.

Perhaps the disciple of John the Baptist saw that Jesus’ disciples did not fast conscientiously and carefully as the Pharisees and themselves, therefore they asked.
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.” (Matthew 9:15)

Jesus stressed the importance of fasting. Once, when Jesus cured a boy who suffered from demonic possession, “Then the disciples approached Jesus in private and said, "Why could we not drive it out?"

He said to them, "Because of your little faith. Amen, I say to you, if you have faith the size of a mustard seed, you will say to this mountain, 'Move from here to there,' and it will move. Nothing will be impossible for you."

"But this kind does not come out except by prayer and fasting"”
(Matthew 17:19-21)
What Jesus emphasizes here is our heart of trusting in Him and humility.