Jeremiah refuses to believe that the Lord has rejected his people forever. It does not fit with who God had revealed himself to be in the history of Israel. Already in the time of Moses, God had promised that an exile would not be the end of his people (Deuteronomy 30:3, Deuteronomy 6:1–25).
Thus, although the book of Lamentations does not end on a joyous note, it does end with a glimmer of hope.
22 unless you have utterly rejected us, and you remain exceedingly angry with us.