That dog is a jerk.
You guys think the dog is the victim but he's not. How is the cat supposed to know that the dog can't have chocolate if the dog never opens himself up.
Maybe the dog shouldn't just say, "Nice try." Maybe he should say, "Cat, I appreciate what you're trying to do here, but you should know something: my species and chocolate just don't mix. Still, I understand your gesture is one of goodwill, so I certainly wouldn't hold it against you."
No, Mr. "I'm the victim" Dog just thinks, "Everyone's out to get me, so clearly this is not a simple gift, but a weapon!" And so he shuts himself off from yet another potential friend. Way to go, dog. NOT!