Maybe you're too emotionally needy in the physical affection department, and he's tired of it? Or, maybe he's just tired. Or depressed, or you've left the honeymoon stage of your relationship, or...
Maybe he's just happy you're near him, and not so happy when you're trying to smother him in an attempt to be physically affectionate? I know that there are times when I find that behavior extremely offputting. "Please please pay attention to me!" When all I want is to just relax and no worry about having to exert energy because I've had a long day at work.
"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."