This is obviously coming from a Dommes experience, but there's something extremely sexy about the "first time experience" with someone. Obviously, you can only get that once with a partner. With a professional, you really don't know exactly what to expect (if you leave your interests open). The anticipation of it, the fear of the unknown, is extremely hot. I feed off that fear/excitement, and it brings out a hunter/primalness that I can't always get with my partner who I've played with over 100x.