Jeremiah 33

Jeremiah 33:21

then My covenant may also be broken with David My servant so that he will not have a son to reign on his throne, and with the Levitical priests, My ministers.

As we often do today, the Israelites had thought of God as a permanent fixture in their culture.  He was always there, and always would bless them, because He had made a covenant with David.  Since He's locked into the deal, why not do whatever we want?

However, they had forgotten that a covenant is two-sided.  Both parties have responsibilities and obligations, which must be fulfilled for the covenant to be honored.  They had broken their part of the deal, and therefore God is telling them that His promise is void.