A long term relationship offers the greatest luxury in life: enough time to spend with each other. There are exciting times, and others that feel like a failure. It requires strategy, effort, and a will to make it work to stay together... a relationship needs maintenance. Blaming the other for routine will only cause a defensive argument, to say at least...
To keep it all exciting, maintain a sense of playfulness. "Sex is the last arena where we can afford to get back to playing as adults." (Esther Perel) You don't have to go places, it's enough if you look at each other with new eyes. Desire needs imagination, thrill needs adventure, and safety needs emotional intimacy.
When you are sure something will be yours forever, you rapidly loose interest... Only fear of loss can make you appreciate your partner again.
Here is one of Esther Perel's amazing TED video presentation that reveals the secret to desire in a long term relationship. She is one of the world's most original and insightful voices on couples and sexuality across cultures.