What is the meaning of a dream of losing a rotten teeth?

Brush your teeth more often...

A serious response:
In dreams, teeth generally symbolize power or potency, as in familiar phrases like "put some teeth into it." Rotten teeth suggest corrupted power, so getting rid of that sort of teeth/power would be beneficial in the long run, even though painful while in the process. Altogether, this dream suggests that the dreamer needs to face the unpleasant and difficult process of getting rid of something corrupt and "rotten" in reality. Note that this is more likely to be about something in the dreamer's personal life rather than on a community or national scale.