I am not an overly assertive person; maybe overly confident sometimes but not necessarily overly assertive. Specially in certain aspects of my life, mainly work.
But, when I am 110% sure I am right and people won't listen to me, I can be an ass. Because after the first half a dozen times, when I've tried explaining politely how I am right, I think I lose the ability to be polite anymore.
I'm not talking about it, I'm just sayin'. (and this line is copied form someplace, can't remember where but it is).