"And no one puts new wine into old containers: Otherwise the wine will break the containers and spill the wine, and the containers will be useless. Rather, new wine must be put into new containers." (Mark 2:22)This parable by Jesus follows the bridechamber parable and the unshrunk garment parable, then this.
Why does he need a third parable?
Because Jesus is trying to communicate a spiritual matter in a way that is understandable to those around him. He is also covering different aspects of the situation. In the bridegroom parable, he discusses his relationship with his students. In the garment parable, he discusses how the teachings coming from the Supreme Being have been passing down over the ages.
Jesus is comparing the teachings of God's representatives to wine. While wine will be virtually the same substance from harvest to harvest, each new harvest winepress will be put into new wine containers.
In the same way, while the teachings of God's representatives will have the same message - as they all represent the Supreme Being - they will be laid out according to the time and circumstance.
Time and circumstance
God's representative will adapt the teachings to the culture and society of those around him, so they may be able to apply those teachings in a practical way to their lives. The core teachings do not change, however.
This is because the teachings of God's representative come from the Supreme Being - who is always the same person, and His wishes for us do not change. He always wants us to return to Him. He wants us to return to our loving service relationship with Him because He loves us and knows that only this will make us happy.
Remember that John baptized Jesus, so technically, he was Jesus' teacher. This means that John's disciples shared the same teacher with Jesus. John the Baptist taught from the desert and John was a strict observer of the disciplines passed on by his teachers. Some of these regarded strict fasting on days commemorating the prophets before him.
Contrasting John, Jesus was teaching among the people of the towns of Judea. The people did not travel out to the desert - a strenuous trip to say the least. Thus Jesus was adapting the message of the Supreme Being to those with less capacity to strictly apply the institutional rules and regulations.
In terms of Jesus' students, Jesus was engaging his students in practical service to the Supreme Being. In this practical service, they had to keep their strength up. This included traveling from town to town in a very hot, desert-like climate; and then organizing Jesus' lectures for him.
For some of his disciples, it also included traveling out in small groups to different towns:
After these events the Master commissioned seventy-two others and dispatched them in pairs in advance of his appearance within the villages and places he was going to go. (Luke 10:1)Because his students were doing practical service - intended to help him preach the message of the Supreme Being - he did not instruct them to fast according to the institutional holidays.
We should also note that most of the fasting holidays observed at that time by institutional custom are either no longer observed today or are observed without fasting.
Jesus taught to please God
The bottom line is that Jesus' intent was to please the Supreme Being in a practical way:
"By myself I can do nothing; I judge only as I hear, and my judgment is just, for I seek not to please myself but Him who sent me." (John 5:30)This central mission - to please the Supreme Being - runs consistent among every prophet and representative of God. It is the Supreme Being who is passing on His message through Jesus:
“My teaching is not my own. It comes from the One who sent me." (John 7:16)And Jesus recognized that he didn't have all the time in the world to get this done for the Supreme Being:
“I am with you for only a short time, and then I am going to the One who sent me." (John 7:33)Thus we can understand that Jesus' focus was to employ his students in his mission as he worked on behalf of the Supreme Being to get the message of the Supreme Being out.
And what was the central teaching of Jesus?
" 'Love the Lord your God with all your heart and with all your soul and with all your mind.' This is the first and greatest commandment. And the second is like it: 'Love your neighbor as yourself.' All the Law and the Prophets hang on these two commandments." (Matt. 22:37-40)