There are many reasons why the alleged victim or reporting party may have a change of heart. It may be that the report of crime (violence, theft, or other violation) may have been false or inaccurate. It may also be that the person is scared to proceed with a prosecution.
The police and the prosecutor’s office are aware of all the reasons, and do not just “drop charges” especially in domestic violence cases. The decision as to whether or not to proceed with the case is up to the prosecutor’s office. They can still proceed even if the “victim” does not want to go forward.