Theophylact of Ohrid

It is the malcontent nature of the Jews that He is speaking of here. For as they were cantankerous, neither John's asceticism nor Christ's simplicity pleased them. They were like foolish little children who are never satisfied - whether one cries for them or plays the pipe for them, they are not pleased.

Commentary on Matthew ↗

Jerome

But to whom shall I compare this generation? It is like children sitting in the marketplace and calling to their playmates, 'We played the flute for you, and you did not dance; we sang a dirge, and you did not mourn.' For John came neither eating nor drinking, and they say, 'He has a demon.' The Son of Man came eating and drinking, and they say, 'Look, a glutton and a drunkard, a friend of tax collectors and sinners.' And wisdom is justified by her children. …

Commentary on Matthew ↗

Hilary of Poitiers

By the children are meant the Prophets, who preached as children in singleness of meaning, and in the midst of the synagogue, that is in the market-place, reprove them, that when they played to those to whom they had devoted the service of their body, they had not obeyed their words, as the movement of the dancers are regulated by the measures of the music. For the Prophets invited them to make confession by song to God, as it is contained in the song of Moses, of Isaiah, or of David.

Catena Aurea by Aquinas ↗

Thomas Aquinas

But to what shall I compare this generation? Here he breaks forth into rebuking the crowd: first, he places a question; secondly, a likeness; thirdly, he explains. He proceeds, therefore, in this manner: John is compared to Elijah by reason of his office, but to what shall I compare this generation? Why does he say this here? …

Commentary on Matthew ↗