Some of the situations you identify (dating, friendship, sales, as well as interviews) have the attribute of "found nothing compelling". There was no single problem, but nothing emerged in the package or the interview that stood out.

In this case, the lack-of-match is not obviously the candidate's fault, and trying to provide feedback is forcing a conclusion ("that's your problem right there") that isn't justified by the information gathered.

And the best thing in such a case may be to say nothing, because you know nothing. ("Whereof one cannot speak, thereof one must be silent.")