Body language!
Interesting question, Max, and so many answers!
What makes the first timer, so terrified at the thought of being inadequate, come back? What keeps the seasoned campaigner coming back to see his favourite girl? And what about the person who only wants to talk?
in every case it's not about what you do or say, but how you say or do it. Flirtation, flattery, touching, subtle body language signals. It doesn't take much to make someone feel special.
Fantasies can be fulfilled, and purely mechanical needs met, but if you can make someone walk (stagger..hehe)away thinking "I think she really liked me!" then you're a bloody expert, and that person will be back.
To paraphrase wiser people before me -
Being good at what you do! hehe (cli-max)
Passion and energy (MASTER WIZARD)
understanding (Hilly)
Do unto me what I would wish you to do unto me! (sorry, that was mine..:3some: )