Another one from Flow: The Psychology of Optimal Experience:
How to keep love fresh? The answer is the same as it is for any other activity. To be enjoyable, a relationship must become more complex. To become more complex, the partners must discover new potentialities in themselves and in each other. To discover these, they must invest attention in each other–so that they can learn what thoughts and feelings, what dreams reside in their partner’s mind. This in itself is a never-ending process, a lifetime’s task. After one begins to really know another person, then many joint adventures become possible: travelling together, reading the same books, raising children, making and realizing plans all become more enjoyable and more meaningful. The specific details are unimportant. Each person must find out which ones are relevant to his or her own situation. What is important is the general principle: that sexuality, like any other aspect of life, can be made enjoyable if we are willing to take control of it, and cultivate it in the direction of greater complexity.