I was in a hotel with a booty call friend once and she paused the activity to get up and forcibly pull the mirror off the wall to move it to a place where it would reflect the action on the bed.
Seeing oneself reflected inspires performance and inventiveness - sex is almost always hotter and more lively with the view. If she's going to rip the damn mirror off the wall, well... the motivation to deliver is multiplied considerably.
I'll never lose those particular images from the spank bank.