Well. This is the simplest, most direct excuse for the dashing-prince-wants-the-unremarkable-heroine that I've seen yet. Not exactly romantic . . .
"It has to be you!"
"Why me, over all those beautiful, well-born women?"
"Well, uh, see, they all faint or throw up when I get too close, so it literally has to be you!"
I'm not sure if I think it's hackneyed laziness or genius.