I'm not a girl, but the major factor that started my initial weight loss was the desire to impress a girl at work.

Take it from me, don't do it. Losing weight to impress another is not a road you want to go down. Yeah, its nice if you end up succeeding in in losing the fat; but that person you're trying to impress may never end up becoming attracted to you, anyway.

Case in point, the girl I had a crush on would have NEVER gone out with me, ever. No matter how I looked, or how skinny I'd gotten. Fact was, I just wasn't her type, and probably never would be. I ended up depressed, frustrated, wondering if my efforts had been in vain. (they hadn't) I was young and stupid at the time, but I've gotten alot wiser.

Don't lose weight to impress others. Do it for yourself. Do it because YOU want it, because you want to be healthier, stronger, better, whatever. Do it for YOU. Not them.