These unbelieving religious leaders did not like John because he was too much of an ascetic. He would not "dance" for them. However they did not like Jesus either. They believed He was too much of a libertine as they defined that term, too joyful. Jesus would not "weep" for them. Because John ate locusts and wild honey instead of bread and wine, the unbelieving Pharisees and lawyers accused him of having a demon. His fanatical behavior also suggested this to them. Jesus, on the other hand, took part in feasts eating and drinking freely. They accused Him of gluttony and drunkenness. The Old Testament described an Israelite who was a glutton and a drunkard as worthy of stoning (cf. Deuteronomy 21:20). Furthermore Jesus associated with people whom the Jewish leaders regarded as apostates. [source][source][source]
"People who want to avoid the truth about themselves can always find something in the preacher to criticize." [1][source]
John and Jesus were both living parables. John taught the importance of repentance, and Jesus offered joy and blessing. However the Jewish religious leaders missed the points of both their messages because John and Jesus did not "dance to their tunes." Jesus probably referred to Himself as the Son of Man here because this title always stresses His deity ( Daniel 7:13-14). This would heighten the seriousness of the religious leaders" rejection. [source][source][source]