"I passed through the stages of grief - denial, anger, pity, resignation and acceptance - in that order and pretty quickly," he said.
Then he set up a website, sorryeverybody.com, to apologise to the world for failing to prevent Mr Bush's victory.
"A lot of people felt maybe they could have done more for the election and were feeling responsible to the rest of the world for the result," he said.
He posted a picture of himself holding a small piece of paper saying: "Sorry world, we tried - (signed) half of America," and inviting other Americans to do the same.
Half joke, half serious, the idea became wildly popular.
Two weeks, 50m hits and more than 15,000 images later the project has proved such a success that he is thinking of turning it into a political lobby group.
"We have to be careful because we don't want to just use it for our own ends," said Mr Zetlen, a neuroscience student at the University of Southern California.
"But we're thinking perhaps making into a PAC [political action committee] or a charity to try to promote a more civil discourse."
If responses to the site are anything to go by, he has a lot of work to do.
More than two-thirds of the images submitted, some from Republicans, were rejected because they were too offensive.
"Some of them absolutely believe that the website is treasonous," he said. "Occasionally they threaten violence."
Others were from Democrats denouncing Bush voters as ignorant. "There's a lot of anger and vilification but we won't accept submissions that are offensive," he said.
Like just about everything else in American politics, one political initiative inevitably produces opposites, surrogates and spin-offs. Six Republican-supporting websites have been created in response: notsorryeverybody.com, sorryeverybodymyass.com, kissmyamericanass.com and werenotsorry.com.
"Anything like this is ripe material for parody," Mr Zetlen said. "I can't blame anybody."
Meanwhile, a handful of supportive websites including apologiesaccepted.com, and notsorrynoteverybody.com have sprung up in its defence.
"The whole thing surprised me," Mr Zetlen said.
"I expected my friends to use it but we've had people sending pictures and sending messages from all over America, the south pole, South Africa, China, everywhere. The messages from abroad have mostly been supportive."
Jay Rayner, an Observer journalist and author of the novel, The Apologist, was less shocked.
He created a website, the-apologist.co.uk, to promote his book about a man who apologises for everything he did wrong and is appointed chief apologist of the United Nations. It has attracted more than 8,000 apologies for almost everything. "There is clearly something unique about the web as a vehicle for apologies," said Rayner, who was expecting just a few dozen submissions.
"It has become like a mass confessional and you can feel that everybody can see it regardless of whether the people you're apologising to actually see it or not."
Sorryeverybody is one of several sites Democrats have set up to keep their spirits up since the election defeat.
One has a fake "official seal" of the Democratic party with the face of a wailing baby. Another has Canadians offering to marry Americans wanting to flee north during the second Bush term.
On a typical page on sorryeverybody.com the contributions ran from belligerent to contrite to funny.
One man is seen holding up a sign saying: "Sorry everybody. Central Mississippi is not all red. I'm one of the 40% who voted for Kerry. Please forgive us."