This one isnt that tough if you know what your positive traits are... just present it in a way that the only reason they would not hire you is if they are not looking for someone with such posive traits.... eg If you are a person who takes leadership roles, you just say that the only reason they wont hire u is if they arent looking for someone who takes leadership roles when needed (and we all know every company is in need of people with such traits)....this not only lets you showcase them what you have but also puts you in a better position without bringing out any negative trait of urself.

" Truthfully, I can't think of single reason, and while I like to be useful and proactive I believe that searching for an answer to that question would not be a productive use of my time"