"Do you have a band-aid? I scraped my knee falling for you."
Translation:Tu aurais un pansement ? Je me suis écorché le genou en tombant amoureux de toi.
I think it's because of the meaning of the whole sentence. In English, to fall for someone means to fall in love with someone. In French, if you say tomber pour toi, it literally means that someone fall in the ground because of someone else (to protect him or whatever). It would be kind of weird.