One explanation that rather floored the person I tried it on, but I think it worked, is that for me being single is my calling. I want to be able to give of myself ... but a nunnery would be the wrong setting. (Not sure about a monastery - I do get along better with men - but institutions and I just do NOT get along for some reason or other, or was it a thousand reasons ).
It kinda befuddled the man, but he's accepted that I'm not interested in his affections, and he'll be off to greener pastures should someone come along that changes my attitude of where I can best be the brightest flower I can...
People *very* rarely question my singleness, though... especially if they know me at all. People do not try to set me up with people; I've asked myself if I"m *that* weird, but ... I think it's just that they can honestly see that I'm happy.
I've also said that there isn't a man out there who's smart enough to keep up with me, and dumb enough to put up with me. It's basically true