&quot;Why?&quot; is a fine response to a statement of intent; but &quot;How do you know that?&quot; as a response to a declarative statement used for motivation can often be more enlightening.<p>(This kind of epistemological work-out is handy in lots of other situations.  When the JWs or Mormons show up at your door, asking &quot;How do you know that?&quot; repeatedly is often all that you need to do until they give up and go away.)