Because so many have been deluded into thinking pleasing a man is everything.  It isn't, YOU come first not them!  In one of my psych classes dealing with sexuality the prof said, "the only person whos sexual gratification you are responsible for is your own."  He was right.  We aren't responsible for theirs, only ours.  It's too bad that most don't realize that and realize how empowering it would be to worry about yourself first and his second!