Counselor: Those who believe that criticism should be gentle rather than harsh should consider the following: change requires a motive, and criticism that is unpleasant provides a motive. Since harsh criticism is unpleasant, harsh criticism provides a motive. Ttherefore, only harsh criticism will cause the person criticized to change.
The reasoning in the counselor's argument is most vulnerable to criticism on the grounds that the argument infers that something that is sufficient to provide a motive is necessary to provide a motive.