Girls are easy as forget, that's why.
almost as easy as they are to forget, am i right ;)
you just have to find that certain seduction "area" which is easy to get once you start spooning on the couch (it's awesome if they have a cut or a sunburn so you can rub it with lotion or something and casually work your way around) and then bam, instant seduction.
but that normally only works on girls that have had love before and haven't done it in a long time.