Quick followup on my previous post, An Illustrated Guide to the NiceGuy™ Loses to Bad Boy Myth
The problem is that even many of the worst "bad boys" think of themselves as "nice guys."
Thus the experience of "nice guys" being left by someone is almost universal.
Meanwhile, surprisingly few men consider themselves to be actually bad guys.
And contrary to myth but conforming to actual common sense, most women aren't particularly attracted to men who are actually are and know they're bad!
With the result that nearly every guy who gets broken up with believes women always leave nice guys.
The law of averages, plus maybe a little selective memory, is all we need in order to "know" the story that "women leave nice guys for bad boys" is "true."