What I think is interesting about this kind of question is WHY is it that when you are the rejected person you see it as "What's wrong with me?" and everyone tells you "nothing is wong with you".
But when you are mentally rejecting men yourself (at least in my case) there definitely is something identifiable "wrong" with them. It can be anything depending on your own personal tastes - too short, too tall, too thin, too fat, a smoker, not educated enough, too stupid, not funny, different sense of humour, bald, too many divorces, class, hair colour etc etc.
Wrong in this sense just means "something present that doesn't do it for me" or "something absent that really does do it for me and is important".
In some cases, I do think it is possible to move from the rejected/not serious interested category into the accepted/now seriously interested - depending on the reasons.
The most common movement I've seen in this category it is the man suddenly becoming interested in two situations (1) he has aged and is more ready to settle down/realises his appeal is waning and he will hook up seriously with a woman he previously wasn't interested in because it now suits him and (2) where the woman has improved how she looks physically - usually lost weight and toned up.
I've also seen it but less common with women becoming interesting in previously "not good enough" men. This category is usually where the man has serious career success and becomes very wealthy. One case I can think of where a man improved his physical appearance.
So to answer your question nothing is "wrong" with you, other than for some reason you don't fit his picture of what he wants for a serious relationship. You are not a "snap" with his mental image. This could be for a million reasons. He may have a strong physical preference for blondes/brunettes/red heads; he may as you suspect want a woman without children; he may not be seriously interested in anyone; he may want a woman with more money. You could torture yourself like this forever.
Fact is you are more into him than he is into you. He matches your picture more than you match his. Unless the base "wrong" reasons are changeable (which they rarely are) cut your losses.