First, you have to know what makes people laugh to begin with. The reason is not exactly cut and dry. Earleywine says that "the incongruity-resolution" model is "one of the dominant theories in humor." Essentially, not being able to see the punchline coming is what makes it funny. Here's an example:
"What's your idea of a perfect date?"
Earleywine told us that joke, but used April 14th as the punchline, so we spruced it up a bit for him. And in his opinion (this opinion is not supported by marijuana researchers, it's just what he believes), "cannabis lowers the threshold for surprise." Will you ever be high enough to find that April 20th joke funny? There's only one way to find out.
Fancy doctor person Dr. Jim Hamblin wrote in a Splitsider article saying that no one's exactly sure why marijuana causes the type of laughter you get when someone's tickling you, even if nothing is funny. Thanks for nothing, science! He also says laughter is known to be contagious -- so if you add up those two factors together, you get a recipe for people thinking Garfield: A Tail of Two Kitties is funny.