I already gave a reply which suggested three things are wrong with that. It's conveniently right there when anyone clocks on your link, but here's a repeat, and I'll add a fourth item:

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?
4. The comparison invites the reader to think about the total benefits of the war, not the benefit to an individual reader. If you are happy about the end of the war based on the total benefits of winning the war, and everyone else is too, that's another form of double-counting

Also, although it may come under #1, that reasoning indicates we should be a lot happier about the invention of fire or agriculture than about the end of World War II.