Every time I have ever pointed out specific things I don't like in answer to "Why won't you date me?" (back when I was available) the guy has used my reply to insist that he will change and beg for another chance. Then I have to say, "No, I don't believe you will ever change in that way, and even if you did it wouldn't be anytime soon, and offering to change yourself for me is really weird." And then he argues that no, he can change right away, it's no trouble, please give him a chance. It's terribly unpleasant. I stopped giving specific answers, and instead said things like, "I guess we just don't have the right chemistry." Actually I think that's a perfectly good and honest answer, and it's the one that's always true even when there's no specific thing I can put my finger on.

I can't pick out exactly what about someone turns me on or doesn't turn me on because it's subconscious, it's my subconscious mind processing a million details all at once, and even when a person does have, say, bad BO, that's just something that I was actually able to notice consciously so I might think of that as The Reason but once they fix their BO, all the other stuff, the millions of details only my subconscious picks up, those will still be there and the person will be pissed that the "fix" didn't work. So I think actually giving a specific reason, or even two or three, is not as honest as just saying chalking it up to "chemistry" (which of course is shorthand for "it's too complex and subconscious to explain").

If I really wanted to try explaining a lack of chemistry, I'd probably be able to do no better than, "Some things about you, especially your para-language but other aspects of your behavior as well, though I can't put my finger on them, rub me the wrong way, or at least inspire no romantic response in me." Would anyone really find that helpful?