When you read this passage to yourself, it helps to imagine the children speaking in a taunting, snotty voice: "We played the flute and you did not dance; we sang a dirge and you did not mourn." Jesus says this unbelieving generation is like those children. Nothing you do satisfies them. Everything you do is subject to their criticism. You did this. How come you didn't do that? You did that. How come you didn't do this? They also feel entitled to set the agenda. You will dance when we say you dance, and you will mourn when we say you mourn.
John the Baptist hailed from the old covenant era. He was the last of a long line of Old Testament prophets who practiced the asceticism and repentance that was appropriate for those who waited for the Messiah's coming. John had put off worldly things. He lived in the wilderness. He dressed in camel's hair. He ate a frugal diet of locusts and honey. In other words he took on the posture of someone preparing to meet his Lord and his Judge because, well, he was. Yet most people saw only a crazy man. So extreme. So out there. Must be demon-possessed or something. "For John came neither eating nor drinking and they say, 'He has a demon!'" The eyes of unbelief are always looking for reasons to reject the messengers of God.
But John was kind of crazy-looking. Are you sure you want to say that people rejected him out of unbelief? John probably looked like the equivalent of today's homeless person shouting out in the street as if he were on drugs. He didn't exactly package his message to appeal to the demographics he was trying to reach.
Fair enough. Enter the Messiah and see how people respond to him. Jesus is not out there wandering in the wilderness but comes straight into town where people live. He's wading through the crowds, he's going to people's weddings, he's making the wine flow, he's the guest of honor in every home that will open its doors to him. That's because his arrival means the time of mourning and waiting and longing is over. The bridegroom is here, let's get this party started. It's a time of celebration, the kingdom of God has arrived. The king himself has arrived. No more of John's extreme practices and extreme diets and extreme everything. Happy now?
No, of course not. Now comes the criticism about the Messiah's overindulgence. "The Son of Man came eating and drinking and they say, 'Behold, a gluttonous man and a drunkard, a friend of tax-gatherers and sinners!'" How can this social butterfly be the Messiah? Where's the self-deprivation of John? Where's the fiery judgment for sinners that John talked about? Huh? Huh? Huh? All of a sudden everyone's so concerned that Jesus is not acting like John, and this becomes an excuse for not believing in him. Never mind the fact that they never believed John in the first place.
Unbelief is like that, too smart for its own good. I'll never be fooled. I see through your tricks. I know what you're up to. Even if I have to contradict myself, it's better than being taken in. But in reality it's acting like children who idly make fun others just because it makes them feel superior. They don't even know what they want, but they do know that they're determined to be unhappy with whatever you give them.