Stick your finger in his ass when you're going down on him...
Basically, you've lost the "OMG SOMEONE NEW!!!!!111!one!11" brain chemicals that you were feeling. Which is much of the "spark" in a relationship. Chances are you've become comfortable with the other person, developed routines, and don't actively change up your lives and patterns enough to keep interested in the person.
Eventually the brain chemicals wear of, the triggers become desensitized, and what you're left with is a real relationship that has real ups and downs, and real life to deal with. It's normal, and pretty much you have to remember not to take the other person's presence in your life for granted.
"Well, then," the Cat went on, "you see a dog growls when it's angry, and wags its tail when it's pleased. Now I growl when I'm pleased, and wag my tail when I'm angry. Therefore I'm mad."