You've hit the nail on the head, Roger. Even when I really like a girl like Stephanie or Kim (Kama), eventually I'm ready to move on. I do like to build a relationship with an MP over several visits, but the day I get the slightest bit bored is the day I go elsewhere.
As far as the life-cycle of an MP goes, I was impressed speaking to Maya at Isis. She's paying her way through school and has serious career plans that don't include massage. Good for her.