It's rude to laugh when someone dies.
John is at home when his dad calls to tell him that his mother has died suddenly.
John wants to react to the bad news.
John tells his father that it's terrible and tells him to hang on since he's on the way over.
John instinctively bursts out laughing thinking that it must be a hoax, and he's being tricked.
John's father feels a bit of comfort from knowing his son will come be with him.
John's father is suddenly taken aback and saddened that John is not taking the horrible news seriously.