Conversely, I have found that when I am interested (none of this applies now that I have been happily married for almost 5 years) and want to start dating, or if I have my eye on a particular man, I become less attractive to them. They want a challenge. My guess is a total shot in the dark, and I could be way off, but I'm thinking there may have been an obvious "I'm not interested" signal that they see as a challenge.

Aggravating, isn't it?