I'm not officially a lawyer, since I haven't passed the Bar Exam yet, but I've recently graduated from Harvard Law School and and I can tell you that underlying the premise of criminal law, is the prerogative that the true "victim" of any criminal activity is not the particular individual harmed by the crime, but rather the whole fabric of society (represented by the state). Therefore, even though a complainant may not wish to see their assailant arrested or tried, it is the state which decides whether it will pursue the matter or not.
Sometimes, as a matter of practicality, if a victim is reluctant to cooperate in the prosecution the state will let the matter drop. However, neither the complainant`s cooperation or consent are legally required for the prosecution and conviction of a criminal offense.