Honestly, alot. GIrls like a good looking man. BUT, we also like sweet, charming, funny, smart, and caring men. So if a not so attractive man has any of these qualities, a girl will be willing to look over the fact that the man isn't attractive.
This answer covers both sides of the spectrum in that attraction, which is trigger by a preference of the physical aspect of people, initializes the whole process. It reels in whoever it can but the more important part is next, and that would be personality which keeps people around (hopefully). I could go further but I think I answered it, heh.