People don't fall in love with hair colors. Sometimes you end up falling for someone with a different hair color, it happens, you can't control it. But yes, for some reason I see more blonde girls with dark haired guys than the other way round. Maybe it has to do with the fact that the majority of girls prefer guys with dark hair.
Two reasons 1. A lot of girls bleach their hair. A lot of those blonde girls are not naturally blonde. 2. Opposites attract. I also have more a thing for darker hair. It's because if the gene pool differs a lot from one another then they have more chance to make strong babies.