I am skeptical of this.

1. We're not happy about the successful conclusion of World War II because it is distant in time, and that seems reasonable unless he's arguing that we should be happier about, say, the death of Genghis Khan.
2. He seems to imply that we should be happy at the end of World War II because the total benefits from winning the war are large. But people were also happy at the intermediate steps of winning the war and that happiness needs to be subtracted. In other words, if you're happy at the liberation of France, you can't be happy at the end of the war based on the entire benefit of winning the war, including the portion of that benefit that consists of France being liberated. That's double counting.
3. This argument would apply to bad news too. Among people who think Obama's Iran deal is likely to lead to Iran getting nuclear weapons, should they be a lot unhappier than they are?