But really, what kind of person can't think of a single reason why maybe they're alone? I could come up with a dozen if I thought for ten minutes. If you genuinely think there is no reason why you are single, you lack the ability (or desire) to see any flaws in yourself. I am not single, but I can still think of lots of reasons why I might/should/could be/was.
That's not the why of it, though. Anybody can think of reasons why they're single. When this discussion comes up, it's always a comparison between the overwhelming chivalry, charm, soul, generosity or other non-romantic aspect of the protagonist and the dastardly, callow, chauvinist and offensive aspects of the antagonist. This comparison is seen as drowning out any other possible discussion - "well yes, I have a weak chin and an inability to hold a job, but he's such an asshole it shouldn't matter that he's a triathlete trial lawyer!"