Jeremiah was very young , he didnt want to be a Prophet, but God had different plans for him.
God sent out Jeremiah to let His people know they were in for a big awakening, they turned their back on God for useless idols and more.
This upset God very much, because of their continues sin against the Lord He even said stop praying for these people , because He wasnt going to listen to his prayers anymore for the people.
Even though God was considering to give them a chance to reconcile, they still kept on with their wicked ways.
So God allowed the Babylonians to destroy Jerusalem.