Don't remember for how long, but part of a year in high school, at the very least. I assume it was mostly non-physical reasons that prompted it, mostly, so I won't go through those, but two things (one which makes for a good anecdote and the second that's just plain too classic not to share):
I never had braces, unlike a *lot* of my classmates (so many, in fact, that I have a fair idea of what living with braces is like anyway just from listening to them!)
I get along well pretty quickly with most animals, and always have. Often works with people, too--I make the acquaintance type of friend very easily if I get to the point of conversation.
That's not to say I'm perfect, and I spent probably a good few hours total trying to convince her that I wasn't a Sue, but there you have it. If I wrote myself as-is (ie, not as Dawn) into a story and didn't show the flaws, I could probably get a Sue pretty easily. It'd be the type of Sue who's relatively harmless but very, very obviously needs more balance. I'd like to think I do have that balance (despite the fact that almost-out-of-university!me has even more Sue traits, without really having lost most of the earlier ones), but, well, not quite the idea of this thread, is it?
(For more detail on how far from me Dawn is by now, and how far from me she was when I came up with her, go ahead and ask. Believe me, I'm more than willing to talk about this, and it'll probably make it into an a/n sometime even if no one asks me here--there actually already is something of the sort in an unfinished interlude wherein she visits her family. No idea when that's ever going to be finished, though.)
~Z, who also keeps being told she has a good singing voice despite a general lack of training. What a world we live in! Then again, Sues do have to come from somewhere, and I could see one part of it being just looking at people, especially celebrities and some classmates, and only seeing the best, most talented parts...