I have it easy. I laugh because someone...usually lots of someones...is trying very hard to be funny. They have witty lines. They've practiced comic timing. And they put enthusiasm into it. So I don't have to think about it. I just laugh.
But did you know there are people who've made laughing a mission? Not comedians, but people like The Levity Project that go into crowded places like Grand Central Station and just start laughing, counting on the sheer contagion...and maybe a sign or two... to get everybody else laughing. And it works! A whole different meaning of Laughing for a Living.