Pain can be something wonderful when

• it happens intentionally,
• when you like pain,
• when the other person can deal with the situation and the given limits.

What I particularly like about this situation is the dynamic between the two of them. She is being beaten, certainly a tolerable pain, but a pain nonetheless. And while she receives it, she does everything to not only increase his arousal by enduring it, but also forces herself to do it elsewhere ;). source