90% of sexual arousal is in your head. Sometimes you're just not sexually compatible, and sometimes you're simply not putting in the effort to make it interesting for you. I've had great sex with friends, and terrible sex with my first wife.
Mostly it's all about effort, mood, and timing. The rest of it depends on what you're into. Make a game out of it, get a blindfold, buy a sex toy together. Find ways to please each other in bed.
"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."