because we have the same back ground. in the case we can understand each other, and I can really know why the joke he tells is funny. the second reason is that he always make a funny voice and amusing gesture when he is kidding. the gesture would let me associate him with monkeys or other animals. and I think a man like an animal is funny, then I burst into laugh. 两个原因顺序换一下更好。 个人见解。