It's good to have a positive outlook, but sometimes it's also just downright annoying. Maybe you don't want to look on the bright side! Some things are actually really bad! Positive people are kind of the worst.
If you're dealing with an overly positive coworker, you've got it pretty bad. But at least you don't have to work with Darryl. He seems...pretty strange.
Here are the Monty Python guys singing about looking on the bright side of life. They are much less annoying than your coworker.
If you want a look into the science of optimism, check out this TED Talk. Maybe that positive guy was onto something after all...