We often see the picture of him weeping because of the fate of Israel. Jeremiah spent much of his time pleading to God and asking him to spare the people. He also tried to get the people to repent of their sins. Notice there are three things we are told not to boast in and then are told what we are to boast in. Knowledge and Understanding of God.
Wisdom can be a good thing.
So teach us to number our days, that we may apply our hearts unto wisdom. We boast in it. Again, what is our wisdom compared to God's. For the wisdom of this world is foolishness with God. For it is written, He taketh the wise in their own craftiness.