Satiation, the thing no one wants to talk about. Relationships become satiated, that is to say, we get used to each other and that means it is harder to stimulate your partner or respond to a partner. It is a natural thing, just as eating steak for every meal would get boring. Successful partners can learn to keep their relationship fresh but there is no template for doing it. Each relationship must find its own way.
The biggest roadblock to creativity in relationships is deeply programmed ideas and assumptions about what is right and wrong, good or bad, healthy or not healthy. Many of these ideas come directly from early religious training. Others from the sex negative messages in our culture. In my experience, the vast majority of people have no idea how much they are impacted by these deeply held notions. They don't even think to question them. As a result, entire aspects of their sexuality and that of their partner's is off limits for even discussion, let alone action. The result is growing dissatisfaction and boredom.
If you can't even talk about something with a trusted partner, there is probably something going on in that area of your life, sexuality or relationship. In Chapter 23 I discuss how we are "Imprisoned by our religious map." Exploring your sexual map, can reveal many hidden areas and assumptions that have the potential to negatively impact any relationship.