Hopefully when you're high you're more of a Louis C.K. than a Louis Vuitton. We're assuming that French designer was a serious dude.
A slightly nerdier explanation
The deputy director at NORML, Paul Armentano, told us, "Cannabinoids, THC in particular, alter mood because they activate receptors that are concentrated in various parts of the brain that regulate mood control." And it makes sense that if marijuana puts you in a better mood, you'll be more likely to laugh. Especially if you smoke weed and watch a video where a kid falls over. That stuff is never not funny. But it's extra funny when you're high.
Since a lot of marijuana's side effects haven't previously been heavily studied by the mainstream scientific community (you know, since it remains illegal, and all), there's no definitive answer out there, yet. Perhaps if it becomes legal federally one day, researchers will provide definitive answers... or at least the government will come clean about all the covert research it's been conducting on us all along! (Next article: why does weed make you think the government is reading your thoughts?)