Chemistry. Sometimes I don't cum after hours of sex. Other time I cum just from cyber sex.
I need dirty, dirty nasty talk to cum. That's how I am. The only exception to this is if the guy is dominant and rough enough that the dirty talk itself is physical. Such as... if the guy uses my pussy for the sole purpose of slicking his cock enough to fuck my ass.. I loose my fucking mind.
But sometimes even with dirty talk and rough sex, I'm just not getting there. I'll enjoy myself a lot, but never past the point of n"O" return.