But what to do when your emotional-balance-and-faith-in-humanity could really use a tune-up, but there's nothing all that funny going on?
Practice laughing. Make it happen on purpose. Doesn't matter whether you're feeling it at first, doesn't matter if it seems like faking it, doesn't matter if it takes a while. Our emotional brain is kind of naive - sorry, but it's true. The cortical processes underlying emotional reactions aren't rational and can be pushed around by what we're exposed to. You already know this, of course - that's why emotions are contagious. Use it as a force for good - practice laughing.
How? Time-honored method - get with someone else who’s laughing. No one like that handy? No problem, there’s always YouTube! Suggestion - go for silly and popular. Silly is good, the emotional brain likes silly. Popularity means easy, and easy is awesome.
Example below. Did it go viral? Heck yeah.